Network Working Group Y. Rekhter
INTERNET DRAFT Juniper Networks
T. Li
Procket Networks, Inc.
S. Hares
NextHop Technologies, Inc.
Editors
A Border Gateway Protocol 4 (BGP-4)
<draft-ietf-idr-bgp4-18.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [RFC2119].
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Table of Contents
Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1. Definition of commonly used terms . . . . . . . . . . . . . . 4
2. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6
3. Summary of Operation . . . . . . . . . . . . . . . . . . . . . 7
3.1 Routes: Advertisement and Storage . . . . . . . . . . . . . . 9
3.2 Routing Information Bases . . . . . . . . . . . . . . . . . . 10
4. Message Formats . . . . . . . . . . . . . . . . . . . . . . . 11
4.1 Message Header Format . . . . . . . . . . . . . . . . . . . . 11
4.2 OPEN Message Format . . . . . . . . . . . . . . . . . . . . . 12
4.3 UPDATE Message Format . . . . . . . . . . . . . . . . . . . . 14
4.4 KEEPALIVE Message Format . . . . . . . . . . . . . . . . . . 21
4.5 NOTIFICATION Message Format . . . . . . . . . . . . . . . . . 21
5. Path Attributes . . . . . . . . . . . . . . . . . . . . . . . 23
5.1 Path Attribute Usage . . . . . . . . . . . . . . . . . . . . 25
5.1.1 ORIGIN . . . . . . . . . . . . . . . . . . . . . . . . . . 25
5.1.2 AS_PATH . . . . . . . . . . . . . . . . . . . . . . . . . . 25
5.1.3 NEXT_HOP . . . . . . . . . . . . . . . . . . . . . . . . . 26
5.1.4 MULTI_EXIT_DISC . . . . . . . . . . . . . . . . . . . . . . 28
5.1.5 LOCAL_PREF . . . . . . . . . . . . . . . . . . . . . . . . 28
5.1.6 ATOMIC_AGGREGATE . . . . . . . . . . . . . . . . . . . . . 29
5.1.7 AGGREGATOR . . . . . . . . . . . . . . . . . . . . . . . . 30
6. BGP Error Handling . . . . . . . . . . . . . . . . . . . . . . 30
6.1 Message Header error handling . . . . . . . . . . . . . . . . 30
6.2 OPEN message error handling . . . . . . . . . . . . . . . . . 31
6.3 UPDATE message error handling . . . . . . . . . . . . . . . . 32
6.4 NOTIFICATION message error handling . . . . . . . . . . . . . 34
6.5 Hold Timer Expired error handling . . . . . . . . . . . . . . 34
6.6 Finite State Machine error handling . . . . . . . . . . . . . 34
6.7 Cease . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
6.8 BGP connection collision detection . . . . . . . . . . . . . 35
7. BGP Version Negotiation . . . . . . . . . . . . . . . . . . . 36
8. BGP Finite State machine . . . . . . . . . . . . . . . . . . . 36
8.1 Events for the BGP FSM . . . . . . . . . . . . . . . . . . . 37
8.1.1 Administrative Events . . . . . . . . . . . . . . . . . . 37
8.1.2 Timer Events . . . . . . . . . . . . . . . . . . . . . . . 38
8.1.3 TCP connection based Events . . . . . . . . . . . . . . . . 39
8.1.4 BGP Messages based Events . . . . . . . . . . . . . . . . . 41
8.2 Description of FSM . . . . . . . . . . . . . . . . . . . . . 43
8.2.1 FSM Definition . . . . . . . . . . . . . . . . . . . . . . 43
8.2.1.1 Terms "active" and "passive" . . . . . . . . . . . . . . 43
8.2.1.2 FSM and collision detection . . . . . . . . . . . . . . . 44
8.2.2 Finite State Machine . . . . . . . . . . . . . . . . . . . 44
9. UPDATE Message Handling . . . . . . . . . . . . . . . . . . . 57
9.1 Decision Process . . . . . . . . . . . . . . . . . . . . . . 58
9.1.1 Phase 1: Calculation of Degree of Preference . . . . . . . 59
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9.1.2 Phase 2: Route Selection . . . . . . . . . . . . . . . . . 60
9.1.2.1 Route Resolvability Condition . . . . . . . . . . . . . . 61
9.1.2.2 Breaking Ties (Phase 2) . . . . . . . . . . . . . . . . . 62
9.1.3 Phase 3: Route Dissemination . . . . . . . . . . . . . . . 64
9.1.4 Overlapping Routes . . . . . . . . . . . . . . . . . . . . 65
9.2 Update-Send Process . . . . . . . . . . . . . . . . . . . . . 66
9.2.1 Controlling Routing Traffic Overhead . . . . . . . . . . . 67
9.2.1.1 Frequency of Route Advertisement . . . . . . . . . . . . 67
9.2.1.2 Frequency of Route Origination . . . . . . . . . . . . . 68
9.2.2 Efficient Organization of Routing Information . . . . . . . 68
9.2.2.1 Information Reduction . . . . . . . . . . . . . . . . . . 68
9.2.2.2 Aggregating Routing Information . . . . . . . . . . . . . 69
9.3 Route Selection Criteria . . . . . . . . . . . . . . . . . . 72
9.4 Originating BGP routes . . . . . . . . . . . . . . . . . . . 72
10. BGP Timers . . . . . . . . . . . . . . . . . . . . . . . . . 72
Appendix A. Comparison with RFC1771 . . . . . . . . . . . . . . . 73
Appendix B. Comparison with RFC1267 . . . . . . . . . . . . . . . 74
Appendix C. Comparison with RFC 1163 . . . . . . . . . . . . . . 75
Appendix D. Comparison with RFC 1105 . . . . . . . . . . . . . . 75
Appendix E. TCP options that may be used with BGP . . . . . . . . 76
Appendix F. Implementation Recommendations . . . . . . . . . . . 76
Appendix F.1 Multiple Networks Per Message . . . . . . . . . . . 76
Appendix F.2 Reducing route flapping . . . . . . . . . . . . . . 77
Appendix F.3 Path attribute ordering . . . . . . . . . . . . . . 77
Appendix F.4 AS_SET sorting . . . . . . . . . . . . . . . . . . . 77
Appendix F.5 Control over version negotiation . . . . . . . . . . 78
Appendix F.6 Complex AS_PATH aggregation . . . . . . . . . . . . 78
Security Considerations . . . . . . . . . . . . . . . . . . . . . 79
References . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
Authors Information . . . . . . . . . . . . . . . . . . . . . . . 80
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Abstract
The Border Gateway Protocol (BGP) is an inter-Autonomous System rout-
ing protocol.
The primary function of a BGP speaking system is to exchange network
reachability information with other BGP systems. This network reacha-
bility information includes information on the list of Autonomous
Systems (ASs) that reachability information traverses. This informa-
tion is sufficient to construct a graph of AS connectivity from which
routing loops may be pruned and some policy decisions at the AS level
may be enforced.
BGP-4 provides a set of mechanisms for supporting Classless Inter-
Domain Routing (CIDR) [RFC1518, RFC1519]. These mechanisms include
support for advertising a set of destinations as an IP prefix and
eliminating the concept of network "class" within BGP. BGP-4 also
introduces mechanisms which allow aggregation of routes, including
aggregation of AS paths.
Routing information exchanged via BGP supports only the destination-
based forwarding paradigm, which assumes that a router forwards a
packet based solely on the destination address carried in the IP
header of the packet. This, in turn, reflects the set of policy deci-
sions that can (and can not) be enforced using BGP. BGP can support
only the policies conforming to the destination-based forwarding
paradigm.
1. Definition of commonly used terms
This section provides definition for terms that have a specific mean-
ing to the BGP protocol and that are used throughout the text.
Autonomous System (AS)
The classic definition of an Autonomous System is a set of routers
under a single technical administration, using an interior gateway
protocol (IGP) and common metrics to determine how to route pack-
ets within the AS, and using an inter-AS routing protocol to
determine how to route packets to other ASs. Since this classic
definition was developed, it has become common for a single AS to
use several IGPs and sometimes several sets of metrics within an
AS. The use of the term Autonomous System here stresses the fact
that, even when multiple IGPs and metrics are used, the adminis-
tration of an AS appears to other ASs to have a single coherent
interior routing plan and presents a consistent picture of what
destinations are reachable through it.
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BGP speaker
A router that implements BGP.
BGP Identifier
A 4-octet unsigned integer indicating the BGP Identifier of the
sender of BGP messages. A given BGP speaker sets the value of its
BGP Identifier to an IP address assigned to that BGP speaker. The
value of the BGP Identifier is determined on startup and is the
same for every local interface and every BGP peer.
Internal peer
Peer that is in the same Autonomous System as the local system.
IBGP
Internal BGP (BGP connection between internal peers).
External peer
Peer that is in a different Autonomous System than the local sys-
tem.
EBGP
External BGP (BGP connection between external peers).
NLRI
Network Layer Reachability Information.
Route
A unit of information that pairs a set of destinations with the
attributes of a path to those destinations. The set of destina-
tions are systems whose IP addresses are contained in one IP
address prefix carried in the Network Layer Reachability Informa-
tion (NLRI) field of an UPDATE message. The path is the informa-
tion reported in the path attributes field of the same UPDATE mes-
sage.
RIB
Routing Information Base.
Adj-RIB-In
The Adj-RIBs-In contain unprocessed routing information that has
been advertised to the local BGP speaker by its peers.
Loc-RIB
The Loc-RIB contains the routes that have been selected by the
local BGP speaker's Decision Process.
Adj-RIB-Out
The Adj-RIBs-Out contains the routes for advertisement to specific
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peers by means of the local speaker's UPDATE messages.
IGP
Interior Gateway Protocol - a routing protocol used to exchange
routing information among routers within a single Autonomous Sys-
tem.
Feasible route
A route that is available for use.
Unfeasible route
A previously advertised feasible route that is no longer available
for use.
2. Acknowledgments
This document was originally published as RFC 1267 in October 1991,
jointly authored by Kirk Lougheed and Yakov Rekhter.
We would like to express our thanks to Guy Almes, Len Bosack, and
Jeffrey C. Honig for their contributions to the earlier version
(BGP-1) of this document.
We would like to specially acknowledge numerous contributions by Den-
nis Ferguson to the earlier version of this document.
We like to explicitly thank Bob Braden for the review of the earlier
version (BGP-2) of this document as well as his constructive and
valuable comments.
We would also like to thank Bob Hinden, Director for Routing of the
Internet Engineering Steering Group, and the team of reviewers he
assembled to review the earlier version (BGP-2) of this document.
This team, consisting of Deborah Estrin, Milo Medin, John Moy, Radia
Perlman, Martha Steenstrup, Mike St. Johns, and Paul Tsuchiya, acted
with a strong combination of toughness, professionalism, and cour-
tesy.
Certain sections of the document borrowed heavily from IDRP
[IS10747], which is the OSI counterpart of BGP. For this credit
should be given to the ANSI X3S3.3 group chaired by Lyman Chapin and
to Charles Kunzinger who was the IDRP editor within that group.
We would also like to thank Benjamin Abarbanel, Enke Chen, Edward
Crabbe, Mike Craren, Vincent Gillet, Eric Gray, Jeffrey Haas, Dimitry
Haskin, John Krawczyk, David LeRoy, Dan Massey, Jonathan Natale, Dan
Pei, Mathew Richardson, John Scudder, John Stewart III, Dave Thaler,
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Paul Traina, Russ White, Curtis Villamizar, and Alex Zinin for their
comments.
We would like to specially acknowledge Andrew Lange for his help in
preparing the final version of this document.
Finally, we would like to thank all the members of the IDR Working
Group for their ideas and support they have given to this document.
3. Summary of Operation
The Border Gateway Protocol (BGP) is an inter-Autonomous System rout-
ing protocol. It is built on experience gained with EGP as defined in
[RFC904] and EGP usage in the NSFNET Backbone as described in
[RFC1092] and [RFC1093].
The primary function of a BGP speaking system is to exchange network
reachability information with other BGP systems. This network reacha-
bility information includes information on the list of Autonomous
Systems (ASs) that reachability information traverses. This informa-
tion is sufficient to construct a graph of AS connectivity from which
routing loops may be pruned and some policy decisions at the AS level
may be enforced.
In the context of this document we assume that a BGP speaker adver-
tises to its peers only those routes that it itself uses (in this
context a BGP speaker is said to "use" a BGP route if it is the most
preferred BGP route and is used in forwarding). All other cases are
outside the scope of this document.
Routing information exchanged via BGP supports only the destination-
based forwarding paradigm, which assumes that a router forwards a
packet based solely on the destination address carried in the IP
header of the packet. This, in turn, reflects the set of policy deci-
sions that can (and can not) be enforced using BGP. Note that some
policies can not be supported by the destination-based forwarding
paradigm, and thus require techniques such as source routing (aka
explicit routing) to be enforced. Such policies can not be enforced
using BGP either. For example, BGP does not enable one AS to send
traffic to a neighboring AS for forwarding to some destination
(reachable through but) beyond that neighboring AS intending that the
traffic take a different route to that taken by the traffic originat-
ing in the neighboring AS (for that same destination). On the other
hand, BGP can support any policy conforming to the destination-based
forwarding paradigm.
A more complete discussion of what policies can and can not be
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enforced with BGP is outside the scope of this document (but refer to
the companion document discussing BGP usage [RFC1772]).
BGP-4 provides a new set of mechanisms for supporting Classless
Inter-Domain Routing (CIDR) [RFC1518, RFC1519]. These mechanisms
include support for advertising a set of destinations as an IP prefix
and eliminating the concept of network "class" within BGP. BGP-4
also introduces mechanisms which allow aggregation of routes, includ-
ing aggregation of AS paths.
This document uses the term `Autonomous System' (AS) throughout. The
classic definition of an Autonomous System is a set of routers under
a single technical administration, using an interior gateway protocol
(IGP) and common metrics to determine how to route packets within the
AS, and using an inter-AS routing protocol to determine how to route
packets to other ASs. Since this classic definition was developed, it
has become common for a single AS to use several IGPs and sometimes
several sets of metrics within an AS. The use of the term Autonomous
System here stresses the fact that, even when multiple IGPs and met-
rics are used, the administration of an AS appears to other ASs to
have a single coherent interior routing plan and presents a consis-
tent picture of what destinations are reachable through it.
The planned use of BGP in the Internet environment, including such
issues as topology, the interaction between BGP and IGPs, and the
enforcement of routing policy rules is presented in a companion docu-
ment [RFC1772]. This document is the first of a series of documents
planned to explore various aspects of BGP application.
BGP uses TCP [RFC793] as its transport protocol. This eliminates the
need to implement explicit update fragmentation, retransmission,
acknowledgment, and sequencing. BGP listens on TCP port 179. Any
authentication scheme used by TCP (e.g., RFC2385 [RFC2385]) may be
used. The error notification mechanism used in BGP assumes that TCP
supports a "graceful" close, i.e., that all outstanding data will be
delivered before the connection is closed.
Two systems form a TCP connection between one another. They exchange
messages to open and confirm the connection parameters.
The initial data flow is the portion of the BGP routing table that is
allowed by the export policy, called the Adj-Ribs-Out (see 3.2).
Incremental updates are sent as the routing tables change. BGP does
not require periodic refresh of the routing table. To allow local
policy changes to have the correct effect without resetting any BGP
connections, a BGP speaker SHOULD either (a) retain the current ver-
sion of the routes advertised to it by all of its peers for the dura-
tion of the connection, or (b) make use of the Route Refresh
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extension [RFC2918].
KEEPALIVE messages may be sent periodically to ensure the liveness of
the connection. NOTIFICATION messages are sent in response to errors
or special conditions. If a connection encounters an error condition,
a NOTIFICATION message is sent and the connection is closed.
The hosts executing BGP need not be routers. A non-routing host
could exchange routing information with routers via EGP [RFC904] or
even an interior routing protocol. That non-routing host could then
use BGP to exchange routing information with a border router in
another Autonomous System. The implications and applications of this
architecture are for further study.
A peer in a different AS is referred to as an external peer, while a
peer in the same AS may be described as an internal peer. Internal
BGP and external BGP are commonly abbreviated IBGP and EBGP.
If a particular AS has multiple BGP speakers and is providing transit
service for other ASs, then care must be taken to ensure a consistent
view of routing within the AS. A consistent view of the interior
routes of the AS is provided by the IGP used within the AS. For the
purpose of this document, it is assumed that a consistent view of the
routes exterior to the AS is provided by having all BGP speakers
within the AS maintain IBGP with each other. Care must be taken to
ensure that the interior routers have all been updated with transit
information before the BGP speakers announce to other ASs that tran-
sit service is being provided.
3.1 Routes: Advertisement and Storage
For the purpose of this protocol, a route is defined as a unit of
information that pairs a set of destinations with the attributes of a
path to those destinations. The set of destinations are systems whose
IP addresses are contained in one IP address prefix carried in the
Network Layer Reachability Information (NLRI) field of an UPDATE mes-
sage, and the path is the information reported in the path attributes
field of the same UPDATE message.
Routes are advertised between BGP speakers in UPDATE messages. Mul-
tiple routes that have the same path attributes can be advertised in
a single UPDATE message by including multiple prefixes in the NLRI
field of the UPDATE message.
Routes are stored in the Routing Information Bases (RIBs): namely,
the Adj-RIBs-In, the Loc-RIB, and the Adj-RIBs-Out, as described in
Section 3.2.
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If a BGP speaker chooses to advertise the route, it may add to or
modify the path attributes of the route before advertising it to a
peer.
BGP provides mechanisms by which a BGP speaker can inform its peer
that a previously advertised route is no longer available for use.
There are three methods by which a given BGP speaker can indicate
that a route has been withdrawn from service:
a) the IP prefix that expresses the destination for a previously
advertised route can be advertised in the WITHDRAWN ROUTES field
in the UPDATE message, thus marking the associated route as being
no longer available for use
b) a replacement route with the same NLRI can be advertised, or
c) the BGP speaker - BGP speaker connection can be closed, which
implicitly removes from service all routes which the pair of
speakers had advertised to each other.
Changing attribute of a route is accomplished by advertising a
replacement route. The replacement route carries new (changed)
attributes and has the same NLRI as the original route.
3.2 Routing Information Bases
The Routing Information Base (RIB) within a BGP speaker consists of
three distinct parts:
a) Adj-RIBs-In: The Adj-RIBs-In store routing information that has
been learned from inbound UPDATE messages received from other BGP
speakers. Their contents represent routes that are available as an
input to the Decision Process.
b) Loc-RIB: The Loc-RIB contains the local routing information
that the BGP speaker has selected by applying its local policies
to the routing information contained in its Adj-RIBs-In. These are
the routes that will be used by the local BGP speaker. The next
hop for each of these routes must be resolvable via the local BGP
speaker's Routing Table.
c) Adj-RIBs-Out: The Adj-RIBs-Out store the information that the
local BGP speaker has selected for advertisement to its peers. The
routing information stored in the Adj-RIBs-Out will be carried in
the local BGP speaker's UPDATE messages and advertised to its
peers.
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In summary, the Adj-RIBs-In contain unprocessed routing information
that has been advertised to the local BGP speaker by its peers; the
Loc-RIB contains the routes that have been selected by the local BGP
speaker's Decision Process; and the Adj-RIBs-Out organize the routes
for advertisement to specific peers by means of the local speaker's
UPDATE messages.
Although the conceptual model distinguishes between Adj-RIBs-In, Loc-
RIB, and Adj-RIBs-Out, this neither implies nor requires that an
implementation must maintain three separate copies of the routing
information. The choice of implementation (for example, 3 copies of
the information vs 1 copy with pointers) is not constrained by the
protocol.
Routing information that the router uses to forward packets (or to
construct the forwarding table that is used for packet forwarding) is
maintained in the Routing Table. The Routing Table accumulates routes
to directly connected networks, static routes, routes learned from
the IGP protocols, and routes learned from BGP. Whether or not a
specific BGP route should be installed in the Routing Table, and
whether a BGP route should override a route to the same destination
installed by another source is a local policy decision, not specified
in this document. Besides actual packet forwarding, the Routing Table
is used for resolution of the next-hop addresses specified in BGP
updates (see Section 5.1.3).
4. Message Formats
This section describes message formats used by BGP.
BGP messages are sent over a TCP connection. A message is processed
only after it is entirely received. The maximum message size is 4096
octets. All implementations are required to support this maximum mes-
sage size. The smallest message that may be sent consists of a BGP
header without a data portion, or 19 octets.
4.1 Message Header Format
Each message has a fixed-size header. There may or may not be a data
portion following the header, depending on the message type. The lay-
out of these fields is shown below:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ +
| Marker |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Marker:
This 16-octet field is included for compatibility; it MUST be
set to all ones.
Length:
This 2-octet unsigned integer indicates the total length of the
message, including the header, in octets. Thus, e.g., it allows
one to locate in the TCP stream the (Marker field of the) next
message. The value of the Length field must always be at least
19 and no greater than 4096, and may be further constrained,
depending on the message type. No "padding" of extra data after
the message is allowed, so the Length field must have the
smallest value required given the rest of the message.
Type:
This 1-octet unsigned integer indicates the type code of the
message. This document defines the following type codes:
1 - OPEN
2 - UPDATE
3 - NOTIFICATION
4 - KEEPALIVE
[RFC2918] defines one more type code.
4.2 OPEN Message Format
After a TCP is established, the first message sent by each side is an
OPEN message. If the OPEN message is acceptable, a KEEPALIVE message
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confirming the OPEN is sent back. Once the OPEN is confirmed, UPDATE,
KEEPALIVE, and NOTIFICATION messages may be exchanged.
In addition to the fixed-size BGP header, the OPEN message contains
the following fields:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+
| Version |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| My Autonomous System |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Hold Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BGP Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opt Parm Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Optional Parameters (variable) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version:
This 1-octet unsigned integer indicates the protocol version
number of the message. The current BGP version number is 4.
My Autonomous System:
This 2-octet unsigned integer indicates the Autonomous System
number of the sender.
Hold Time:
This 2-octet unsigned integer indicates the number of seconds
that the sender proposes for the value of the Hold Timer. Upon
receipt of an OPEN message, a BGP speaker MUST calculate the
value of the Hold Timer by using the smaller of its configured
Hold Time and the Hold Time received in the OPEN message. The
Hold Time MUST be either zero or at least three seconds. An
implementation may reject connections on the basis of the Hold
Time. The calculated value indicates the maximum number of
seconds that may elapse between the receipt of successive
KEEPALIVE, and/or UPDATE messages by the sender.
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BGP Identifier:
This 4-octet unsigned integer indicates the BGP Identifier of
the sender. A given BGP speaker sets the value of its BGP Iden-
tifier to an IP address assigned to that BGP speaker. The
value of the BGP Identifier is determined on startup and is the
same for every local interface and every BGP peer.
Optional Parameters Length:
This 1-octet unsigned integer indicates the total length of the
Optional Parameters field in octets. If the value of this field
is zero, no Optional Parameters are present.
Optional Parameters:
This field may contain a list of optional parameters, where
each parameter is encoded as a <Parameter Type, Parameter
Length, Parameter Value> triplet.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...
| Parm. Type | Parm. Length | Parameter Value (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...
Parameter Type is a one octet field that unambiguously identi-
fies individual parameters. Parameter Length is a one octet
field that contains the length of the Parameter Value field in
octets. Parameter Value is a variable length field that is
interpreted according to the value of the Parameter Type field.
[RFC2842] defines the Capabilities Optional Parameter.
The minimum length of the OPEN message is 29 octets (including mes-
sage header).
4.3 UPDATE Message Format
UPDATE messages are used to transfer routing information between BGP
peers. The information in the UPDATE message can be used to construct
a graph describing the relationships of the various Autonomous Sys-
tems. By applying rules to be discussed, routing information loops
and some other anomalies may be detected and removed from inter-AS
routing.
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An UPDATE message is used to advertise feasible routes sharing common
path attribute to a peer, or to withdraw multiple unfeasible routes
from service (see 3.1). An UPDATE message may simultaneously adver-
tise a feasible route and withdraw multiple unfeasible routes from
service. The UPDATE message always includes the fixed-size BGP
header, and also includes the other fields as shown below (note, some
of the shown fields may not be present in every UPDATE message):
+-----------------------------------------------------+
| Withdrawn Routes Length (2 octets) |
+-----------------------------------------------------+
| Withdrawn Routes (variable) |
+-----------------------------------------------------+
| Total Path Attribute Length (2 octets) |
+-----------------------------------------------------+
| Path Attributes (variable) |
+-----------------------------------------------------+
| Network Layer Reachability Information (variable) |
+-----------------------------------------------------+
Withdrawn Routes Length:
This 2-octets unsigned integer indicates the total length of
the Withdrawn Routes field in octets. Its value must allow the
length of the Network Layer Reachability Information field to
be determined as specified below.
A value of 0 indicates that no routes are being withdrawn from
service, and that the WITHDRAWN ROUTES field is not present in
this UPDATE message.
Withdrawn Routes:
This is a variable length field that contains a list of IP
address prefixes for the routes that are being withdrawn from
service. Each IP address prefix is encoded as a 2-tuple of the
form <length, prefix>, whose fields are described below:
+---------------------------+
| Length (1 octet) |
+---------------------------+
| Prefix (variable) |
+---------------------------+
The use and the meaning of these fields are as follows:
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a) Length:
The Length field indicates the length in bits of the IP
address prefix. A length of zero indicates a prefix that
matches all IP addresses (with prefix, itself, of zero
octets).
b) Prefix:
The Prefix field contains an IP address prefix followed by
enough trailing bits to make the end of the field fall on an
octet boundary. Note that the value of trailing bits is
irrelevant.
Total Path Attribute Length:
This 2-octet unsigned integer indicates the total length of the
Path Attributes field in octets. Its value must allow the
length of the Network Layer Reachability field to be determined
as specified below.
A value of 0 indicates that no Network Layer Reachability
Information field is present in this UPDATE message.
Path Attributes:
A variable length sequence of path attributes is present in
every UPDATE message, except for an UPDATE message that carries
only the withdrawn routes. Each path attribute is a triple
<attribute type, attribute length, attribute value> of variable
length.
Attribute Type is a two-octet field that consists of the
Attribute Flags octet followed by the Attribute Type Code
octet.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attr. Flags |Attr. Type Code|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The high-order bit (bit 0) of the Attribute Flags octet is the
Optional bit. It defines whether the attribute is optional (if
set to 1) or well-known (if set to 0).
The second high-order bit (bit 1) of the Attribute Flags octet
is the Transitive bit. It defines whether an optional attribute
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is transitive (if set to 1) or non-transitive (if set to 0).
For well-known attributes, the Transitive bit must be set to 1.
(See Section 5 for a discussion of transitive attributes.)
The third high-order bit (bit 2) of the Attribute Flags octet
is the Partial bit. It defines whether the information con-
tained in the optional transitive attribute is partial (if set
to 1) or complete (if set to 0). For well-known attributes and
for optional non-transitive attributes the Partial bit must be
set to 0.
The fourth high-order bit (bit 3) of the Attribute Flags octet
is the Extended Length bit. It defines whether the Attribute
Length is one octet (if set to 0) or two octets (if set to 1).
The lower-order four bits of the Attribute Flags octet are
unused. They must be zero when sent and must be ignored when
received.
The Attribute Type Code octet contains the Attribute Type Code.
Currently defined Attribute Type Codes are discussed in Section
5.
If the Extended Length bit of the Attribute Flags octet is set
to 0, the third octet of the Path Attribute contains the length
of the attribute data in octets.
If the Extended Length bit of the Attribute Flags octet is set
to 1, then the third and the fourth octets of the path
attribute contain the length of the attribute data in octets.
The remaining octets of the Path Attribute represent the
attribute value and are interpreted according to the Attribute
Flags and the Attribute Type Code. The supported Attribute Type
Codes, their attribute values and uses are the following:
a) ORIGIN (Type Code 1):
ORIGIN is a well-known mandatory attribute that defines the
origin of the path information. The data octet can assume
the following values:
Value Meaning
0 IGP - Network Layer Reachability Information
is interior to the originating AS
1 EGP - Network Layer Reachability Information
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learned via the EGP protocol [RFC904]
2 INCOMPLETE - Network Layer Reachability
Information learned by some other means
Usage of this attribute is defined in 5.1.1.
b) AS_PATH (Type Code 2):
AS_PATH is a well-known mandatory attribute that is composed
of a sequence of AS path segments. Each AS path segment is
represented by a triple <path segment type, path segment
length, path segment value>.
The path segment type is a 1-octet long field with the fol-
lowing values defined:
Value Segment Type
1 AS_SET: unordered set of ASs a route in the
UPDATE message has traversed
2 AS_SEQUENCE: ordered set of ASs a route in
the UPDATE message has traversed
The path segment length is a 1-octet long field containing
the number of ASs (not the number of octets) in the path
segment value field.
The path segment value field contains one or more AS num-
bers, each encoded as a 2-octets long field.
Usage of this attribute is defined in 5.1.2.
c) NEXT_HOP (Type Code 3):
This is a well-known mandatory attribute that defines the IP
address of the border router that should be used as the next
hop to the destinations listed in the Network Layer Reacha-
bility Information field of the UPDATE message.
Usage of this attribute is defined in 5.1.3.
d) MULTI_EXIT_DISC (Type Code 4):
This is an optional non-transitive attribute that is a four
octet non-negative integer. The value of this attribute may
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be used by a BGP speaker's decision process to discriminate
among multiple entry points to a neighboring autonomous sys-
tem.
Usage of this attribute is defined in 5.1.4.
e) LOCAL_PREF (Type Code 5):
LOCAL_PREF is a well-known attribute that is a four octet
non-negative integer. A BGP speaker uses it to inform other
internal peers of the advertising speaker's degree of pref-
erence for an advertised route.
Usage of this attribute is defined in 5.1.5.
f) ATOMIC_AGGREGATE (Type Code 6)
ATOMIC_AGGREGATE is a well-known discretionary attribute of
length 0.
Usage of this attribute is defined in 5.1.6.
g) AGGREGATOR (Type Code 7)
AGGREGATOR is an optional transitive attribute of length 6.
The attribute contains the last AS number that formed the
aggregate route (encoded as 2 octets), followed by the IP
address of the BGP speaker that formed the aggregate route
(encoded as 4 octets). This should be the same address as
the one used for the BGP Identifier of the speaker.
Usage of this attribute is defined in 5.1.7.
Network Layer Reachability Information:
This variable length field contains a list of IP address pre-
fixes. The length in octets of the Network Layer Reachability
Information is not encoded explicitly, but can be calculated
as:
UPDATE message Length - 23 - Total Path Attributes Length -
Withdrawn Routes Length
where UPDATE message Length is the value encoded in the fixed-
size BGP header, Total Path Attribute Length and Withdrawn
Routes Length are the values encoded in the variable part of
the UPDATE message, and 23 is a combined length of the fixed-
size BGP header, the Total Path Attribute Length field and the
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Withdrawn Routes Length field.
Reachability information is encoded as one or more 2-tuples of
the form <length, prefix>, whose fields are described below:
+---------------------------+
| Length (1 octet) |
+---------------------------+
| Prefix (variable) |
+---------------------------+
The use and the meaning of these fields are as follows:
a) Length:
The Length field indicates the length in bits of the IP
address prefix. A length of zero indicates a prefix that
matches all IP addresses (with prefix, itself, of zero
octets).
b) Prefix:
The Prefix field contains an IP address prefix followed by
enough trailing bits to make the end of the field fall on an
octet boundary. Note that the value of the trailing bits is
irrelevant.
The minimum length of the UPDATE message is 23 octets -- 19 octets
for the fixed header + 2 octets for the Withdrawn Routes Length + 2
octets for the Total Path Attribute Length (the value of Withdrawn
Routes Length is 0 and the value of Total Path Attribute Length is
0).
An UPDATE message can advertise at most one set of path attributes,
but multiple destinations, provided that the destinations share these
attributes. All path attributes contained in a given UPDATE message
apply to all destinations carried in the NLRI field of the UPDATE
message.
An UPDATE message can list multiple routes to be withdrawn from ser-
vice. Each such route is identified by its destination (expressed as
an IP prefix), which unambiguously identifies the route in the con-
text of the BGP speaker - BGP speaker connection to which it has been
previously advertised.
An UPDATE message might advertise only routes to be withdrawn from
service, in which case it will not include path attributes or Network
Layer Reachability Information. Conversely, it may advertise only a
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feasible route, in which case the WITHDRAWN ROUTES field need not be
present.
An UPDATE message should not include the same address prefix in the
WITHDRAWN ROUTES and Network Layer Reachability Information fields,
however a BGP speaker MUST be able to process UPDATE messages in this
form. A BGP speaker should treat an UPDATE message of this form as if
the WITHDRAWN ROUTES doesn't contain the address prefix.
4.4 KEEPALIVE Message Format
BGP does not use any TCP-based keep-alive mechanism to determine if
peers are reachable. Instead, KEEPALIVE messages are exchanged
between peers often enough as not to cause the Hold Timer to expire.
A reasonable maximum time between KEEPALIVE messages would be one
third of the Hold Time interval. KEEPALIVE messages MUST NOT be sent
more frequently than one per second. An implementation MAY adjust the
rate at which it sends KEEPALIVE messages as a function of the Hold
Time interval.
If the negotiated Hold Time interval is zero, then periodic KEEPALIVE
messages MUST NOT be sent.
KEEPALIVE message consists of only message header and has a length of
19 octets.
4.5 NOTIFICATION Message Format
A NOTIFICATION message is sent when an error condition is detected.
The BGP connection is closed immediately after sending it.
In addition to the fixed-size BGP header, the NOTIFICATION message
contains the following fields:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error code | Error subcode | Data (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Error Code:
This 1-octet unsigned integer indicates the type of
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NOTIFICATION. The following Error Codes have been defined:
Error Code Symbolic Name Reference
1 Message Header Error Section 6.1
2 OPEN Message Error Section 6.2
3 UPDATE Message Error Section 6.3
4 Hold Timer Expired Section 6.5
5 Finite State Machine Error Section 6.6
6 Cease Section 6.7
Error subcode:
This 1-octet unsigned integer provides more specific informa-
tion about the nature of the reported error. Each Error Code
may have one or more Error Subcodes associated with it. If no
appropriate Error Subcode is defined, then a zero (Unspecific)
value is used for the Error Subcode field.
Message Header Error subcodes:
1 - Connection Not Synchronized.
2 - Bad Message Length.
3 - Bad Message Type.
OPEN Message Error subcodes:
1 - Unsupported Version Number.
2 - Bad Peer AS.
3 - Bad BGP Identifier.
4 - Unsupported Optional Parameter.
5 - Authentication Failure.
6 - Unacceptable Hold Time.
UPDATE Message Error subcodes:
1 - Malformed Attribute List.
2 - Unrecognized Well-known Attribute.
3 - Missing Well-known Attribute.
4 - Attribute Flags Error.
5 - Attribute Length Error.
6 - Invalid ORIGIN Attribute
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8 - Invalid NEXT_HOP Attribute.
9 - Optional Attribute Error.
10 - Invalid Network Field.
11 - Malformed AS_PATH.
Data:
This variable-length field is used to diagnose the reason for
the NOTIFICATION. The contents of the Data field depend upon
the Error Code and Error Subcode. See Section 6 below for more
details.
Note that the length of the Data field can be determined from
the message Length field by the formula:
Message Length = 21 + Data Length
The minimum length of the NOTIFICATION message is 21 octets (includ-
ing message header).
5. Path Attributes
This section discusses the path attributes of the UPDATE message.
Path attributes fall into four separate categories:
1. Well-known mandatory.
2. Well-known discretionary.
3. Optional transitive.
4. Optional non-transitive.
Well-known attributes must be recognized by all BGP implementations.
Some of these attributes are mandatory and must be included in every
UPDATE message that contains NLRI. Others are discretionary and may
or may not be sent in a particular UPDATE message.
All well-known attributes must be passed along (after proper updat-
ing, if necessary) to other BGP peers.
In addition to well-known attributes, each path may contain one or
more optional attributes. It is not required or expected that all BGP
implementations support all optional attributes. The handling of an
unrecognized optional attribute is determined by the setting of the
Transitive bit in the attribute flags octet. Paths with unrecognized
transitive optional attributes should be accepted. If a path with
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unrecognized transitive optional attribute is accepted and passed
along to other BGP peers, then the unrecognized transitive optional
attribute of that path must be passed along with the path to other
BGP peers with the Partial bit in the Attribute Flags octet set to 1.
If a path with recognized transitive optional attribute is accepted
and passed along to other BGP peers and the Partial bit in the
Attribute Flags octet is set to 1 by some previous AS, it is not set
back to 0 by the current AS. Unrecognized non-transitive optional
attributes must be quietly ignored and not passed along to other BGP
peers.
New transitive optional attributes may be attached to the path by the
originator or by any other BGP speaker in the path. If they are not
attached by the originator, the Partial bit in the Attribute Flags
octet is set to 1. The rules for attaching new non-transitive
optional attributes will depend on the nature of the specific
attribute. The documentation of each new non-transitive optional
attribute will be expected to include such rules. (The description of
the MULTI_EXIT_DISC attribute gives an example.) All optional
attributes (both transitive and non-transitive) may be updated (if
appropriate) by BGP speakers in the path.
The sender of an UPDATE message should order path attributes within
the UPDATE message in ascending order of attribute type. The receiver
of an UPDATE message must be prepared to handle path attributes
within the UPDATE message that are out of order.
The same attribute can not appear more than once within the Path
Attributes field of a particular UPDATE message.
The mandatory category refers to an attribute which must be present
in both IBGP and EBGP exchanges if NLRI are contained in the UPDATE
message. Attributes classified as optional for the purpose of the
protocol extension mechanism may be purely discretionary, or discre-
tionary, required, or disallowed in certain contexts.
attribute EBGP IBGP
ORIGIN mandatory mandatory
AS_PATH mandatory mandatory
NEXT_HOP mandatory mandatory
MULTI_EXIT_DISC discretionary discretionary
LOCAL_PREF see section 5.1.5 required
ATOMIC_AGGREGATE see section 5.1.6 and 9.1.4
AGGREGATOR discretionary discretionary
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5.1 Path Attribute Usage
The usage of each BGP path attributes is described in the following
clauses.
5.1.1 ORIGIN
ORIGIN is a well-known mandatory attribute. The ORIGIN attribute
shall be generated by the speaker that originates the associated
routing information. Its value SHOULD NOT be changed by any other
speaker.
5.1.2 AS_PATH
AS_PATH is a well-known mandatory attribute. This attribute identi-
fies the autonomous systems through which routing information carried
in this UPDATE message has passed. The components of this list can be
AS_SETs or AS_SEQUENCEs.
When a BGP speaker propagates a route which it has learned from
another BGP speaker's UPDATE message, it shall modify the route's
AS_PATH attribute based on the location of the BGP speaker to which
the route will be sent:
a) When a given BGP speaker advertises the route to an internal
peer, the advertising speaker shall not modify the AS_PATH
attribute associated with the route.
b) When a given BGP speaker advertises the route to an external
peer, then the advertising speaker shall update the AS_PATH
attribute as follows:
1) if the first path segment of the AS_PATH is of type
AS_SEQUENCE, the local system shall prepend its own AS number
as the last element of the sequence (put it in the leftmost
position). If the act of prepending will cause an overflow in
the AS_PATH segment, i.e. more than 255 ASs, it shall be legal
to prepend a new segment of type AS_SEQUENCE and prepend its
own AS number to this new segment.
2) if the first path segment of the AS_PATH is of type AS_SET,
the local system shall prepend a new path segment of type
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AS_SEQUENCE to the AS_PATH, including its own AS number in that
segment.
When a BGP speaker originates a route then:
a) the originating speaker shall include its own AS number in a
path segment of type AS_SEQUENCE in the AS_PATH attribute of all
UPDATE messages sent to an external peer. (In this case, the AS
number of the originating speaker's autonomous system will be the
only entry the path segment, and this path segment will be the
only segment in the AS_PATH attribute).
b) the originating speaker shall include an empty AS_PATH
attribute in all UPDATE messages sent to internal peers. (An
empty AS_PATH attribute is one whose length field contains the
value zero).
Whenever the modification of the AS_PATH attribute calls for includ-
ing or prepending the AS number of the local system, the local system
may include/prepend more than one instance of its own AS number in
the AS_PATH attribute. This is controlled via local configuration.
5.1.3 NEXT_HOP
The NEXT_HOP is a well-known mandatory attribute that defines the IP
address of the border router that should be used as the next hop to
the destinations listed in the UPDATE message. The NEXT_HOP attribute
is calculated as follows.
1) When sending a message to an internal peer, if the route is not
locally originated the BGP speaker should not modify the NEXT_HOP
attribute, unless it has been explicitly configured to announce
its own IP address as the NEXT_HOP. When announcing a locally
originated route to an internal peer, the BGP speaker should use
as the NEXT_HOP the interface address of the router through which
the announced network is reachable for the speaker; if the route
is directly connected to the speaker, or the interface address of
the router through which the announced network is reachable for
the speaker is the internal peer's address, then the BGP speaker
should use for the NEXT_HOP attribute its own IP address (the
address of the interface that is used to reach the peer).
2) When sending a message to an external peer X, and the peer is
one IP hop away from the speaker:
- If the route being announced was learned from an internal
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peer or is locally originated, the BGP speaker can use for the
NEXT_HOP attribute an interface address of the internal peer
router (or the internal router) through which the announced
network is reachable for the speaker, provided that peer X
shares a common subnet with this address. This is a form of
"third party" NEXT_HOP attribute.
- Otherwise, if the route being announced was learned from an
external peer, the speaker can use in the NEXT_HOP attribute an
IP address of any adjacent router (known from the received
NEXT_HOP attribute) that the speaker itself uses for local
route calculation, provided that peer X shares a common subnet
with this address. This is a second form of "third party"
NEXT_HOP attribute.
- Otherwise, if the external peer to which the route is being
advertised shares a common subnet with one of the announcing
router's own interfaces, the router may use the IP address
associated with such an interface in the NEXT_HOP attribute.
This is known as a "first party" NEXT_HOP attribute.
- By default (if none of the above conditions apply), the BGP
speaker should use in the NEXT_HOP attribute the IP address of
the interface that the speaker uses to establish the BGP con-
nection to peer X.
3) When sending a message to an external peer X, and the peer is
multiple IP hops away from the speaker (aka "multihop EBGP"):
- The speaker may be configured to propagate the NEXT_HOP
attribute. In this case when advertising a route that the
speaker learned from one of its peers, the NEXT_HOP attribute
of the advertised route is exactly the same as the NEXT_HOP
attribute of the learned route (the speaker just doesn't modify
the NEXT_HOP attribute).
- By default, the BGP speaker should use in the NEXT_HOP
attribute the IP address of the interface that the speaker uses
to establish the BGP connection to peer X.
Normally the NEXT_HOP attribute is chosen such that the shortest
available path will be taken. A BGP speaker must be able to support
disabling advertisement of third party NEXT_HOP attributes to handle
imperfectly bridged media.
A BGP speaker must never advertise an address of a peer to that peer
as a NEXT_HOP, for a route that the speaker is originating. A BGP
speaker must never install a route with itself as the next hop.
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The NEXT_HOP attribute is used by the BGP speaker to determine the
actual outbound interface and immediate next-hop address that should
be used to forward transit packets to the associated destinations.
The immediate next-hop address is determined by performing a recur-
sive route lookup operation for the IP address in the NEXT_HOP
attribute using the contents of the Routing Table, selecting one
entry if multiple entries of equal cost exist. The Routing Table
entry which resolves the IP address in the NEXT_HOP attribute will
always specify the outbound interface. If the entry specifies an
attached subnet, but does not specify a next-hop address, then the
address in the NEXT_HOP attribute should be used as the immediate
next-hop address. If the entry also specifies the next-hop address,
this address should be used as the immediate next-hop address for
packet forwarding.
5.1.4 MULTI_EXIT_DISC
The MULTI_EXIT_DISC is an optional non-transitive attribute which may
be used on external (inter-AS) links to discriminate among multiple
exit or entry points to the same neighboring AS. The value of the
MULTI_EXIT_DISC attribute is a four octet unsigned number which is
called a metric. All other factors being equal, the exit point with
lower metric should be preferred. If received over EBGP, the
MULTI_EXIT_DISC attribute MAY be propagated over IBGP to other BGP
speakers within the same AS. The MULTI_EXIT_DISC attribute received
from a neighboring AS MUST NOT be propagated to other neighboring
ASs.
A BGP speaker MUST IMPLEMENT a mechanism based on local configuration
which allows the MULTI_EXIT_DISC attribute to be removed from a
route. This MAY be done prior to determining the degree of preference
of the route and performing route selection (decision process phases
1 and 2).
An implementation MAY also (based on local configuration) alter the
value of the MULTI_EXIT_DISC attribute received over EBGP. This MAY
be done prior to determining the degree of preference of the route
and performing route selection (decision process phases 1 and 2). See
section 9.1.2.2 for necessary restricts on this.
5.1.5 LOCAL_PREF
LOCAL_PREF is a well-known attribute that SHALL be included in all
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UPDATE messages that a given BGP speaker sends to the other internal
peers. A BGP speaker SHALL calculate the degree of preference for
each external route based on the locally configured policy, and
include the degree of preference when advertising a route to its
internal peers. The higher degree of preference MUST be preferred. A
BGP speaker shall use the degree of preference learned via LOCAL_PREF
in its decision process (see section 9.1.1).
A BGP speaker MUST NOT include this attribute in UPDATE messages that
it sends to external peers, except for the case of BGP Confederations
[RFC3065]. If it is contained in an UPDATE message that is received
from an external peer, then this attribute MUST be ignored by the
receiving speaker, except for the case of BGP Confederations
[RF3065].
5.1.6 ATOMIC_AGGREGATE
ATOMIC_AGGREGATE is a well-known discretionary attribute.
When a router aggregates several routes for the purpose of advertise-
ment to a particular peer, the AS_PATH of the aggregated route nor-
mally includes an AS_SET formed from the set of AS from which the
aggregate was formed. In many cases the network administrator can
determine that the aggregate can safely be advertised without the
AS_SET and not form route loops.
If an aggregate excludes at least some of the AS numbers present in
the AS_PATH of the routes that are aggregated as a result of dropping
the AS_SET, the aggregated route, when advertised to the peer, SHOULD
include the ATOMIC_AGGREGATE attribute.
A BGP speaker that receives a route with the ATOMIC_AGGREGATE
attribute SHOULD NOT remove the attribute from the route when propa-
gating it to other speakers.
A BGP speaker that receives a route with the ATOMIC_AGGREGATE
attribute MUST NOT make any NLRI of that route more specific (as
defined in 9.1.4) when advertising this route to other BGP speakers.
A BGP speaker that receives a route with the ATOMIC_AGGREGATE
attribute needs to be cognizant of the fact that the actual path to
destinations, as specified in the NLRI of the route, while having the
loop-free property, may not be the path specified in the AS_PATH
attribute of the route.
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5.1.7 AGGREGATOR
AGGREGATOR is an optional transitive attribute which may be included
in updates which are formed by aggregation (see Section 9.2.2.2). A
BGP speaker which performs route aggregation may add the AGGREGATOR
attribute which shall contain its own AS number and IP address. The
IP address should be the same as the BGP Identifier of the speaker.
6. BGP Error Handling.
This section describes actions to be taken when errors are detected
while processing BGP messages.
When any of the conditions described here are detected, a NOTIFICA-
TION message with the indicated Error Code, Error Subcode, and Data
fields is sent, and the BGP connection is closed, unless it is
explicitly stated that no NOTIFICATION message is to be sent and the
BGP connection is not to be closed. If no Error Subcode is specified,
then a zero must be used.
The phrase "the BGP connection is closed" means that the TCP connec-
tion has been closed, the associated Adj-RIB-In has been cleared, and
that all resources for that BGP connection have been deallocated.
Entries in the Loc-RIB associated with the remote peer are marked as
invalid. The fact that the routes have become invalid is passed to
other BGP peers before the routes are deleted from the system.
Unless specified explicitly, the Data field of the NOTIFICATION mes-
sage that is sent to indicate an error is empty.
6.1 Message Header error handling.
All errors detected while processing the Message Header are indicated
by sending the NOTIFICATION message with Error Code Message Header
Error. The Error Subcode elaborates on the specific nature of the
error.
The expected value of the Marker field of the message header is all
ones. If the Marker field of the message header is not as expected,
then a synchronization error has occurred and the Error Subcode is
set to Connection Not Synchronized.
If the Length field of the message header is less than 19 or greater
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than 4096, or if the Length field of an OPEN message is less than the
minimum length of the OPEN message, or if the Length field of an
UPDATE message is less than the minimum length of the UPDATE message,
or if the Length field of a KEEPALIVE message is not equal to 19, or
if the Length field of a NOTIFICATION message is less than the mini-
mum length of the NOTIFICATION message, then the Error Subcode is set
to Bad Message Length. The Data field contains the erroneous Length
field.
If the Type field of the message header is not recognized, then the
Error Subcode is set to Bad Message Type. The Data field contains the
erroneous Type field.
6.2 OPEN message error handling.
All errors detected while processing the OPEN message are indicated
by sending the NOTIFICATION message with Error Code OPEN Message
Error. The Error Subcode elaborates on the specific nature of the
error.
If the version number contained in the Version field of the received
OPEN message is not supported, then the Error Subcode is set to
Unsupported Version Number. The Data field is a 2-octets unsigned
integer, which indicates the largest locally supported version number
less than the version the remote BGP peer bid (as indicated in the
received OPEN message), or if the smallest locally supported version
number is greater than the version the remote BGP peer bid, then the
smallest locally supported version number.
If the Autonomous System field of the OPEN message is unacceptable,
then the Error Subcode is set to Bad Peer AS. The determination of
acceptable Autonomous System numbers is outside the scope of this
protocol.
If the Hold Time field of the OPEN message is unacceptable, then the
Error Subcode MUST be set to Unacceptable Hold Time. An implementa-
tion MUST reject Hold Time values of one or two seconds. An imple-
mentation MAY reject any proposed Hold Time. An implementation which
accepts a Hold Time MUST use the negotiated value for the Hold Time.
If the BGP Identifier field of the OPEN message is syntactically
incorrect, then the Error Subcode is set to Bad BGP Identifier. Syn-
tactic correctness means that the BGP Identifier field represents a
valid IP host address.
If one of the Optional Parameters in the OPEN message is not
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recognized, then the Error Subcode is set to Unsupported Optional
Parameters.
If one of the Optional Parameters in the OPEN message is recognized,
but is malformed, then the Error Subcode is set to 0 (Unspecific).
6.3 UPDATE message error handling.
All errors detected while processing the UPDATE message are indicated
by sending the NOTIFICATION message with Error Code UPDATE Message
Error. The error subcode elaborates on the specific nature of the
error.
Error checking of an UPDATE message begins by examining the path
attributes. If the Withdrawn Routes Length or Total Attribute Length
is too large (i.e., if Withdrawn Routes Length + Total Attribute
Length + 23 exceeds the message Length), then the Error Subcode is
set to Malformed Attribute List.
If any recognized attribute has Attribute Flags that conflict with
the Attribute Type Code, then the Error Subcode is set to Attribute
Flags Error. The Data field contains the erroneous attribute (type,
length and value).
If any recognized attribute has Attribute Length that conflicts with
the expected length (based on the attribute type code), then the
Error Subcode is set to Attribute Length Error. The Data field con-
tains the erroneous attribute (type, length and value).
If any of the mandatory well-known attributes are not present, then
the Error Subcode is set to Missing Well-known Attribute. The Data
field contains the Attribute Type Code of the missing well-known
attribute.
If any of the mandatory well-known attributes are not recognized,
then the Error Subcode is set to Unrecognized Well-known Attribute.
The Data field contains the unrecognized attribute (type, length and
value).
If the ORIGIN attribute has an undefined value, then the Error Sub-
code is set to Invalid Origin Attribute. The Data field contains the
unrecognized attribute (type, length and value).
If the NEXT_HOP attribute field is syntactically incorrect, then the
Error Subcode is set to Invalid NEXT_HOP Attribute. The Data field
contains the incorrect attribute (type, length and value). Syntactic
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correctness means that the NEXT_HOP attribute represents a valid IP
host address.
The IP address in the NEXT_HOP must meet the following criteria to be
considered semantically correct:
a) It must not be the IP address of the receiving speaker
b) In the case of an EBGP where the sender and receiver are one IP
hop away from each other, either the IP address in the NEXT_HOP
must be the sender's IP address (that is used to establish the BGP
connection), or the interface associated with the NEXT_HOP IP
address must share a common subnet with the receiving BGP speaker.
If the NEXT_HOP attribute is semantically incorrect, the error should
be logged, and the route should be ignored. In this case, no NOTIFI-
CATION message should be sent, and connection should not be closed.
The AS_PATH attribute is checked for syntactic correctness. If the
path is syntactically incorrect, then the Error Subcode is set to
Malformed AS_PATH.
If the UPDATE message is received from an external peer, the local
system MAY check whether the leftmost AS in the AS_PATH attribute is
equal to the autonomous system number of the peer than sent the mes-
sage. If the check determines that this is not the case, the Error
Subcode is set to Malformed AS_PATH.
If an optional attribute is recognized, then the value of this
attribute is checked. If an error is detected, the attribute is dis-
carded, and the Error Subcode is set to Optional Attribute Error.
The Data field contains the attribute (type, length and value).
If any attribute appears more than once in the UPDATE message, then
the Error Subcode is set to Malformed Attribute List.
The NLRI field in the UPDATE message is checked for syntactic valid-
ity. If the field is syntactically incorrect, then the Error Subcode
is set to Invalid Network Field.
If a prefix in the NLRI field is semantically incorrect (e.g., an
unexpected multicast IP address), an error should be logged locally,
and the prefix should be ignored.
An UPDATE message that contains correct path attributes, but no NLRI,
shall be treated as a valid UPDATE message.
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6.4 NOTIFICATION message error handling.
If a peer sends a NOTIFICATION message, and the receiver of the mes-
sage detects an error in that message, the receiver can not use a
NOTIFICATION message to report this error back to the peer. Any such
error, such as an unrecognized Error Code or Error Subcode, should be
noticed, logged locally, and brought to the attention of the adminis-
tration of the peer. The means to do this, however, lies outside the
scope of this document.
6.5 Hold Timer Expired error handling.
If a system does not receive successive KEEPALIVE and/or UPDATE
and/or NOTIFICATION messages within the period specified in the Hold
Time field of the OPEN message, then the NOTIFICATION message with
Hold Timer Expired Error Code must be sent and the BGP connection
closed.
6.6 Finite State Machine error handling.
Any error detected by the BGP Finite State Machine (e.g., receipt of
an unexpected event) is indicated by sending the NOTIFICATION message
with Error Code Finite State Machine Error.
6.7 Cease.
In absence of any fatal errors (that are indicated in this section),
a BGP peer may choose at any given time to close its BGP connection
by sending the NOTIFICATION message with Error Code Cease. However,
the Cease NOTIFICATION message must not be used when a fatal error
indicated by this section does exist.
A BGP speaker may support the ability to impose an (locally config-
ured) upper bound on the number of address prefixes the speaker is
willing to accept from a neighbor. When the upper bound is reached,
the speaker (under control of local configuration) may either (a)
discard new address prefixes from the neighbor (while maintaining BGP
connection with the neighbor), or (b) terminate the BGP connection
with the neighbor. If the BGP speaker decides to terminate its BGP
connection with a neighbor because the number of address prefixes
received from the neighbor exceeds the locally configured upper
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bound, then the speaker must send to the neighbor a NOTIFICATION mes-
sage with the Error Code Cease.
6.8 BGP connection collision detection.
If a pair of BGP speakers try simultaneously to establish a BGP con-
nection to each other, then two parallel connections between this
pair of speakers might well be formed. If the source IP address used
by one of these connections is the same as the destination IP address
used by the other, and the destination IP address used by the first
connection is the same as the source IP address used by the other, we
refer to this situation as connection collision. Clearly in the
presence of connection collision, one of these connections must be
closed.
Based on the value of the BGP Identifier a convention is established
for detecting which BGP connection is to be preserved when a colli-
sion does occur. The convention is to compare the BGP Identifiers of
the peers involved in the collision and to retain only the connection
initiated by the BGP speaker with the higher-valued BGP Identifier.
Upon receipt of an OPEN message, the local system must examine all of
its connections that are in the OpenConfirm state. A BGP speaker may
also examine connections in an OpenSent state if it knows the BGP
Identifier of the peer by means outside of the protocol. If among
these connections there is a connection to a remote BGP speaker whose
BGP Identifier equals the one in the OPEN message, and this connec-
tion collides with the connection over which the OPEN message is
received then the local system performs the following collision reso-
lution procedure:
1. The BGP Identifier of the local system is compared to the BGP
Identifier of the remote system (as specified in the OPEN mes-
sage). Comparing BGP Identifiers is done by treating them as
(4-octet long) unsigned integers.
2. If the value of the local BGP Identifier is less than the
remote one, the local system closes BGP connection that already
exists (the one that is already in the OpenConfirm state), and
accepts BGP connection initiated by the remote system.
3. Otherwise, the local system closes newly created BGP connection
(the one associated with the newly received OPEN message), and
continues to use the existing one (the one that is already in the
OpenConfirm state).
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Unless allowed via configuration, a connection collision with an
existing BGP connection that is in Established state causes closing
of the newly created connection.
Note that a connection collision can not be detected with connections
that are in Idle, or Connect, or Active states.
Closing the BGP connection (that results from the collision resolu-
tion procedure) is accomplished by sending the NOTIFICATION message
with the Error Code Cease.
7. BGP Version Negotiation
BGP speakers may negotiate the version of the protocol by making mul-
tiple attempts to open a BGP connection, starting with the highest
version number each supports. If an open attempt fails with an Error
Code OPEN Message Error, and an Error Subcode Unsupported Version
Number, then the BGP speaker has available the version number it
tried, the version number its peer tried, the version number passed
by its peer in the NOTIFICATION message, and the version numbers that
it supports. If the two peers do support one or more common versions,
then this will allow them to rapidly determine the highest common
version. In order to support BGP version negotiation, future versions
of BGP must retain the format of the OPEN and NOTIFICATION messages.
8. BGP Finite State machine
This section specifies the BGP operation in terms of a Finite State
Machine (FSM). The section falls into 2 parts:
1) Description of Events for the State machine (section 8.1)
2) Description of the FSM (section 8.2)
Session Attributes required for each connection are;
1) State
2) Connect Retry timer
3) Hold timer
4) Hold time
5) Keepalive timer
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8.1 Events for the BGP FSM
8.1.1 Administrative Events
Please note that only Event 1 (manual start) and Event 2 (manual
stop) are mandatory administrative events. All other administrative
events are optional.
Event1: Manual start
Definition: Administrator manually starts peer
connection.
Status: Mandatory
Event2: Manual stop
Definition: Local system administrator manually
stops the peer connection.
Status: Mandatory
Event3: Automatic start
Definition: Local system automatically starts the
BGP connection.
Status: Optional depending on local system
Event4: Manual start with passive TCP establishment
Definition: Administrator manually start the peer
connection, but has the passive flag
enabled. The passive flag indicates
that the peer will listen prior to
establishing the connection.
Status: Optional depending on local system
Event5: Automatic start with passive TCP establishment
Definition: Local system automatically starts the
BGP connection with the passive flag
enabled. The passive flag indicates
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that the peer will listen prior to
establishing a connection.
Status: Optional depending on local system use
of a passive connection.
Event6: Automatic start with bgp_stop_flap option set
Definition: Local system automatically starts the
BGP peer connection with persistent peer
oscillation damping enabled. The exact
method of damping persistent peer
oscillations is left up to the
implementation. These methods of
damping persistent BGP adjacency
flapping are outside the scope of this
document.
Status: Optional, used only if the bgp peer has
Enabled a method of damping persistent
BGP peer flapping.
Event7: Auto stop
Definition: Local system automatically stops the
BGP connection.
Status: Optional depending on local system
8.1.2 Timer Events
Event8: Idle hold timer expires
Definition: Idle Hold timer expires. The Idle
Hold Timer is only used when persistent
BGP oscillation damping functions are
enabled.
Status: Optional. Used when persistent
BGP peer oscillation damping functions
are enabled.
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Event9: Connect retry timer expires
Definition: An event triggered by the expiration of
the ConnectRetry timer.
Status: Mandatory
Event10: Hold time expires
Definition: An event generated when the HoldTimer
expires.
Status: Mandatory
Event11: Keepalive timer expires
Definition: A periodic event generated due to the
expiration of the KeepAlive Timer.
Status: Mandatory
Event12: DelayBGP open timer expires
Definition: A timer that delays sending of the BGP
Open message for n seconds after the
TCP connection has been completed.
Status: Optional
8.1.3 TCP Connection based Events
Event13: TCP connection indication & valid remote peer
Definition: Event indicating that TCP connection
request with a valid source IP address and TCP
port, and valid destination IP address
and TCP Port. The definition of
invalid source, and invalid destination
IP address is left to the implementation.
BGP's destination port should be port
179 as defined by IANA.
TCP connection request is denoted by
the local system receiving a TCP SYN.
Status: Mandatory
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Event14: RCV TCP connection indication with invalid source or
destination
Definition: TCP connection request received with either
an invalid source address or port
number or an invalid destination
address or port number. BGP destination
port number should be 179 as defined
by IANA.
Again, a TCP connection request is is
denoted by local system receiving a TCP
SYN with an invalid source port or
destination address or port number.
Status: Mandatory
Event15: TCP connection request sent received an ACK.
Definition: Local system's request to establish a TCP
connection to the remote side received
an ACK.
The local system's TCP session sent a TCP
SYN, and received a TCP SYN, ACK pair of
messages, and Sent a TCP ACK.
Status: Mandatory
Event16: TCP connection confirmed
Definition: The local system has received a confirmation that
the TCP connection has been established by
the remote site.
The remote peer's TCP engine sent a TCP SYN.
The local peer's TCP engine sent a SYN, ACK
pair, and now has received a final ACK.
Status: Mandatory
Event17: TCP connection fails
Definition: This BGP peer receives a TCP
connection failure notice.
The remote BGP peer's TCP machine could have
sent a FIN. The local peer would respond
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with a FIN-ACK. Another alternative is that
the local peer indicated a timeout in the
TCP session and downed the connection.
Status: Mandatory
8.1.4 BGP Messages based Events
Event18: BGPOpen
Definition: An event indicating that a valid Open
message has been received.
Status: Mandatory
Event19: BGPOpen with BGP Delay Open Timer running
Definition: An event indicating that a valid Open
message has been successful
established for a peer that is
currently delaying the sending of an
BGP Open message.
Status: Optional
Event20: BGPHeaderErr
Definition: BGP message header is not valid.
Status: Mandatory
Event21: BGPOpenMsgErr
Definition: An BGP Open message has been received
with errors.
Status: Mandatory
Event22: Open collision dump
Definition: An event generated administratively
when a connection Collision has been
detected while processing an incoming
Open message. This connection has been
selected to disconnected. See section
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6.8 for more information on collision
detection.
Event 22 is an administrative could
occur if FSM is implemented as two
linked state machines.
Status: Optional
Event23: NotifMsgVerErr
Definition: An event is generated when a
NOTIFICIATION message with "version
error" is received.
Status: Mandatory
Event24: NotifMsg
Definition: An event is generated when a
NOTIFICATION messages is received and
the error code is anything but
"version error".
Status: Mandatory
Event25: KeepAliveMsg
Definition: An event is generated when a KEEPALIVE
message is received.
Status: Mandatory
Event26: UpdateMsg
Definition: An event is generated when a valid
Update message is received.
Status: Mandatory
Event27: UpdateMsgErr
Definition: An event is generated when an invalid
Update message is received.
Status: Mandatory
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8.2 Description of FSM
8.2.1 FSM Definition
BGP must maintain a separate FSM for each configured peer, Each BGP
peer paired in a potential connection unless configured to remain in
the idle state, or configured to remain passive, will attempt to to
connect to the other. For the purpose of this discussion, the active
or connect side of the TCP connection (the side of a TCP connection
(the side sending the first TCP SYN packet) is called outgoing. The
passive or listening side (the sender of the first SYN ACK) is called
an incoming connection. [See section on the terms active and passive
below.]
A BGP implementation must connect to and listen on TCP port 179 for
incoming connections in addition to trying to connect to peers. For
each incoming connection, a state machine must be instantiated.
There exists a period in which the identity of the peer on the other
end of an incoming connection is known but the BGP identifier is not
known. During this time, both an incoming and an outgoing connection
for the same configured peering may exist. This is referred to as a
connection collision (see Section x.x, was 6.8).
A BGP implementation will have at most one FSM for each configured
peering plus one FSM for each incoming TCP connection for which the
peer has not yet been identified. Each FSM corresponds to exactly one
TCP connection.
There may be more than one connections between a pair of peers if the
connections are configured to use a different pair of IP addresses.
This is referred to as multiple "configured peerings" to the same
peer.
8.2.1.1 Terms "active" and "passive"
The terms active and passive have been in our vocabulary for almost a
decade and have proven useful. The words active and passive have
slightly different meanings applied to a TCP connection or applied to
a peer. There is only one active side and one passive side to any
one TCP connection per the definition above and the state machine
below. When a BGP speaker is configured active it may end up on
either the active or passive side of the connection that eventually
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gets established. Once the TCP connection is completed, it doesn't
matter which end was active and which end was passive and the only
difference is which side of the TCP connection has port number 179.
8.2.1.2 FSM and collision detection
There is one FSM per BGP connection. Prior to determining what peer
a connection is associated with there may be two connections for a
given peer. There should be no more than one connection per peer.
The collision detection identifies the case where there is more than
one connection per peer and provides guidance for which connection to
get rid of. When this occurs, the corresponding FSM for the connec-
tion that is closed should be disposed of
8.2.2 Finite State Machine
Idle state:
Initially BGP is in the Idle state.
In this state BGP refuses all incoming BGP connections. No
resources are allocated to the peer. In response to a
manual start event(Event1) or an automatic start
event(Event3), the local system
- initializes all BGP resources,
- sets ConnectRetryCnt (the connect retry counter) to zero
- starts the connect retry timer with initial value,
- initiates a TCP connection to the other BGP peer,
- listens for a connection that may be initiated by
the remote BGP peer, and
- changes its state to connect.
An manual stop event (Event2) is ignored in the Idle state.
In response to a manual start event with the passive TCP connection
flag (Event 4) or automatic start with the passive TCP connection
flag (Event 5), the local system:
- initializes all BGP resources,
- sets ConnectRetryCnt (the connect retry counter) to zero,
- start the connect retry timer with initial value,
- listens for a connection that may be initiated by
the remote peer, and
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- changes its state to Active.
The exact value of the ConnectRetry timer is a local
matter, but it should be sufficiently large to allow TCP
initialization.
If a persistent BGP peer oscillation damping function is
enabled, two additional events may occur within Idle state:
- Automatic start with bgp_stop_flap set [Event6],
- Idle Hold Timer expired [Event 8].
The method of preventing persistent BGP peer oscillation is
outside the scope of this document.
Any other events [Events 9-27] received in the Idle state,
are noted by the MIB processing as FSM Errors
and the local peer stays in the Idle State.
Connect State:
In this state, BGP is waiting for the TCP connection to
be completed.
If the TCP connection succeeds [Event 15 or
Event 16], the local system checks the "Delay Open
Flag". If the delay Open flag is set, the local system:
- clears the connect retry timer,
- set the BGP open delay timer to the initial
value.
If the Delay Open flag is not set, the local system:
- clears the connect retry timer,
- completes BGP initialization
- send an Open message to its peer,
- sets hold timer to a large value, and
- Change the state to Open Sent.
A hold timer value of 4 minutes is suggested.
If the Open Delay timer expires [Event 12] in the connect
state,
- send an Open message to its peer,
- set the hold timer to a large value, and
- change the state to Open Sent.
If the BGP port receives a TCP connection indication
[Event 13], the TCP connection is processed and
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the connection remains in the connected state.
If the TCP connection receives an indication
that is invalid or unconfigured. [Event 14]:
- the TCP connection is rejected.
If the TCP connection fails (timeout or disconnect)
[Event17], the local system:
- restarts the connect retry timer,
- continues to listen for a connection that may be
initiated by the remote BGP peer, and
- changes its state to Active.
If an Open is received with the BGP Delay Open timer is
running [Event 19], the local system:
- clears the connect retry timer (cleared to zero),
- completes the BGP initialization,
- Stops and clears the BGP Open Delay timer
- Sends an Open message
- Set the hold timer to a large value (4 minutes), and
- changes its state to Open Confirm.
The start events [Event 1, 3-6] are ignored in connect
state.
A manual stop event[Event2], the local system:
- drops the TCP connection,
- releases all BGP resources,
- sets ConnectRetryCnt (the connect retry count) to zero
- resets the connect retry timer (sets to zero), and
- goes to Idle state.
In response to the connect retry timer expired event(Event
9), the local system:
- Sets the MIB FSM error information with connect retry
expired,
- drops the TCP connection
- restarts the connect retry timer
- initiates a TCP connection to the other BGP
peer,
- continues to listen for a connection that may be
initiated by the remote BGP peer, and
- stays in Connect state.
In response to any other events [Events 7-8, 10-11, 18, 20-
27] the local system:
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- resets the connect retry timer (sets to zero),
- drops the TCP connection,
- release all BGP resources,
- increments the ConnectRetryCnt (connect retry count) by 1,
- [optionally] performs bgp peer oscillation damping, and
- goes to Idle state.
Active State:
In this state BGP is trying to acquire a peer by listening
for and accepting a TCP connection.
A TCP connection succeeds [Event 15 or Event 16], the
local system: process the TCP connection flags
- If the BGP delay open flag is set:
o clears the connect retry timer,
o completes the BGP initialization, and
o sets the BGP delay Open timer
- If the BGP delay open flag is not set:
o clears the connect retry timer,
o completes the BGP initialization,
o sends the Open message to it's peer,
o sets its hold timer to a large value,
and changes its state to OpenSent.
A Hold timer value of 4 minutes is suggested.
If the local system receives a valid TCP Indication
[Event 13], the local system processes the TCP connection flags.
If the local system receives a TCP indication
that is invalid for this connection [Event 14]:
- the TCP connection is rejected.
If the local system receives a TCP connection
failed [Event 17] (timeout or receives connection
disconnect), the local system will:
- set TCP disconnect in the MIB reason code,
- restart connect retry timer (with initial value)
- release all BGP resources
- Acknowledge the drop of TCP connection if
TCP disconnect (send a FIN ACK),
- Increment ConnectRetryCnt (connect retry count) by 1, and
- perform the BGP peer oscillation damping process [2].
If the local system has the delay open timer expired [event
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12] local system:
- clears the connect retry timer (set to zero),
- stops and clears the delay open timer (set to zero)
- completes the BGP initialization,
- sends the Open message to it's remote peer,
- sets its hold timer to a large value,
- and set the state to Open Confirm.
A hold timer value of 4 minutes is also suggested for this
state transition.
If an Open is received with the BGP delay open timer is
running [Event 19], the local system
- clears the connect retry timer (cleared to zero),
- stops and clears the BGP open delay timer
- completes the BGP initialization,
- stops and clears the BGP open delay timer
- sends an Open message
- set its hold timer to a large value (4 minutes), and
- changes its state to Open Confirm.
In response the ConnectRetry timer expired event[Event9],
the local system:
- restarts the connect retry timer (with initial value),
- initiates a TCP connection to the other BGP
peer,
- Continues to listen for TCP connection that may be
initiated by remote BGP peer,
- and changes its state to Connect.
The start events [Event1, 3-6] are ignored in the Active
state.
A manual stop event[Event2], the local system:
- Sets the administrative down in the MIB reason code,
- Sends a Notification with a Cease,
- If any BGP routes exist, delete the routes
- release all BGP resources,
- drops the TCP connection,
- sets ConnectRetryCnt (connect retry count) to zero
- resets the connect retry timer (sets to zero),
- goes to Idle state.
In response to any other event (Events 7-8, 10-11,18, 20-
27), the local system:
- stores the MIB information to indicate appropriate
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error [FSM for Events 7-8, 10-11, 18, 20-27]
- reset the connect retry timer (sets to zero),
- release all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCnt (connect retry count) by one,
- optionally performs BGP peer oscillation damping,
- and goes to the idle state
Open Sent:
In this state BGP waits for an Open Message from its peer.
When an OPEN message is received, all fields are checked
for correctness. If there are no errors in the OPEN message
[Event 18] the local system:
- resets the BGP Delay timer to zero,
- reset BGP Connect Timer to zero,
- sends a KEEPALIVE message and
- sets a KeepAlive timer (via the text below)
- sets the Hold timer according to the negotiated value
(see section 4.2), and
- sets the state to Open Confirm.
If the negotiated Hold time value is zero, then the Hold
and KeepAlive timers are not started. If the
value of the Autonomous System field is the same as the
local Autonomous System number, then the connection is an
"internal" connection; otherwise, it is an "external"
connection. (This will impact UPDATE processing as
described below.)
If the BGP message header checking [Event20] or OPEN message
check detects an error (see Section 6.2)[Event21], the local system:
- sends a NOTIFICATION message with appropriate error
code,
- reset the connect retry timer (sets to zero),
- if there are any routes associated with the BGP session,
delete these routes
- release all BGP resources,
- drop the TCP connection
- increments the ConnectRetryCnt (connect retry cout) by 1,
- bgp peer oscillation damping process,
- and goes to the Idle state.
Collision detection mechanisms (section 6.8) need to be
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applied when a valid BGP Open is received [Event 18 or
Event 19]. Please refer to section 6.8 for the details of
the comparison. An administrative collision detect is when
BGP implementation determines my means outside the scope of
this document that a connection collision has occurred.
If a connection in Open Sent is determined to be the
connection that must be closed, an administrative collision
detect [Event 22] is signaled to the state machine. If such
an administrative collision detect dump [Event 22] is
received in Open Sent, the local system:
- sets MIB state information to
collision detect closure,
- send a NOTIFICATION with a CEASE
- resets the connect retry timer,
- release all BGP resources,
- drop the TCP connection,
- increments ConnectRetryCnt (connect rery count) by 1,
- performs any BGP peer oscillation damp process, and
- enters Idle state.
If a NOTIFICATION message is received with a version
error[Event23], Notification message without version number
[Event 24], the local system:
- resets the connect retry timer (sets to zero)
- drops the TCP connection,
- releases all BGP resources,
- increments the ConnectRetryCnt (connect retry count) by 1
- process any BGP peer oscillation damping,
- and sets the state to Idle.
The Start events [Event1, 3-6] are ignored in the OpenSent
state.
If a manual stop event [Event 2] is issued in Open sent
state, the local system:
- Sets administrative down reason in MIB reason,
- sends the Notification with a cease,
- if BGP routes exists, delete the routes,
- Release all BGP resources,
- Drops the TCP connection,
- set ConnectRetryCnt (connect retry count) to zero,
- resets the Connect Retry timer (set to zero), and
- transitions to the Idle state.
If an automatic stop event [Event 7] is issued in Open sent
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state, the local system:
- Sets administrative down reason in MIB reason,
- sends the Notification with a cease,
- if any routes are associated with te BGP session,
delete the routes,
- release all the BGP resources
- Drops the TCP connection,
- increments the ConnectRetryCnt (connect retry count) by 1,
- BGP peer oscillation process [2], and
- transitions to the Idle state.
If the Hold Timer expires[Event 10], the local system:
- set Hold timer expired in MIB Error reason code,
- send a NOTIFICATION message with error code Hold
Timer Expired,
- reset the connect retry timer (sets to zero),
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCnt (connect retry count) by 1,
and transitions to the Idle state.
If a TCP indication is received for valid connection
[Event 13] or TCP request aknowledgement [Event 15]
is received, or a TCP connect confirm [Event 16] is
received a second TCP session may be in progress. This
second TCP session is tracked per the Call Collision
processing (section 6.8) until an OPEN message is received.
A TCP connection for an invalid port [Event 14] is ignored.
If a TCP connection failure [Event17], is received
the local system:
- closes the BGP connection,
- restarts the Connect Retry timer,
- and continues to listen for a connection that may be
initiated by the remote BGP peer,
- and goes into Active state.
In response to any other event [Events 8-9, 11-12, 19, 25-27],
the local system:
- sends the NOTIFICATION with the Error Code Finite
state machine error,
- resets the connect retry timer (sets to zero),
- releases all BGP resources
- drops the TCP connection,
- increments the ConnectRetryCnt (connect retry count) by 1,
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- process any bgp peer oscillation damping[2],
- and sets the state to idle.
Open Confirm State:
In this state BGP waits for a KEEPALIVE or NOTIFICATION
message.
If the local system receives a KEEPALIVE message[Event 25],
- restarts the Hold timer, and
- changes its state to Established.
If the local system receives a NOTIFICATION message [Event
23-24] or receives a TCP Disconnect [Event 17] from the
underlying TCP , the local system:
- sets the appropriate MIB information for FSM error,
- resets the connect retry timer (sets the timer to
zero),
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCnt (connect retry count) by 1,
- and sets the state to idle.
Any start event [Event1, 3-6] is ignored in the OpenConfirm
state.
In response to a manual stop event[Event 2] initiated by
the operator, the local system:
- set Administrative down in MIB Reason code,
- sends the NOTIFICATION message with Cease,
- if any BGP routes, dete the routes
- releases all BGP resources,
- drop the TCP connection,
- sets the ConnectRetryCnt (connect retry count) to zero
- sets the connect retry timer to zero, and
- transitions to Idle state.
In response to the Automatic stop event initiated by the
system[Event 7], the local system:
- sets the MIB entry for this peer to administratively
down,
- sends the NOTIFICATION message with Cease,
- connect retry timer reset (set to zero)
- If any BGP routes exist, delete the routes,
- release all BGP resources,
- drops the TCP connection,
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- increments the ConnectRetryCnt (connect retry count)
by 1, and
- transitions to the Idle State.
If the Hold Timer expires before a KEEPALIVE message is
received [Event 10], the local system:
- set the MIB reason to Hold time expired,
- send the NOTIFICATION message with the error code
set to Hold Time Expired,
- resets the connect retry timer (sets the timer to to
zero),
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCnt (connect retry count) by 1,
- and sets the state to Idle.
If the local system receives a KEEPALIVE timer expires
event [Event 11], the system:
- sends a KEEPALIVE message,
- restarts the Keepalive timer, and
- remains in Open Confirmed state.
In the event of TCP establishment [Event 13], or TCP
connection succeeding [Event 15 or Event 16] while in Open
Confirm, the local system needs to track the 2nd
connection.
If a TCP connection is attempted to an invalid port [Event
14], the local system will ignore the second connection
attempt.
If an OPEN message is received, all fields are check for
correctness. If the BGP message header checking [Event20]
or OPEN message check detects an error (see Section
6.2)[Event21], the local system:
- sends a NOTIFICATION message with appropriate error
code,
- resets the connect retry timer (sets the timer to
zero),
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCnt (connect retry count) by 1,
- runs the BGP peer oscillation damping process [2]
- and goes to the Idle state.
If the Open messages is valid [Event 18], the collision
detect function is processed per section 6.8. If this
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connection is to be dropped due to call collision, the
local system:
- sets the Call Collision cease in the MIB reason
code,
- sends a Notification with a Cease
- resets the Connect timer (set to zero),
- releases all BGP resources,
- Drops the TCP connection (send TCP FIN),
- increments the ConnectRetryCnt by 1 (connect retry count), and
- performs any BGP peer oscillation damping process [2].
If during the processing of another Open message, the BGP
implementation determines my means outside the scope of
this document that a connection collision has occurred and
this connection is to be closed, the local system will
issue a call collision dump [Event 22]. When the local
system receives a call collision dump event [Event 22], the
local system:
- Sets the MIB FSM variable to indicate collision
detected and dump connection.
- send a NOTIFICATION with a CEASE
- deletes all routes associated with connection,
- resets the connect retry timer,
- releases all BGP resources
- drops all TCP connection,
- increments the ConnectRetryCnt (connect retry count) by 1,
- and performs any BGP peer oscillation damping, and
- enters Idle state.
In response to any other event [Events 8-9, 12, 19, 26-27],
the local system:
- sends a NOTIFICATION with a code of Finite State
Machine Error,
- resets the connect retry timer (sets to zero)
- drops the TCP connection,
- releases all BGP resources,
- increments the ConnectRetryCnt (connect retrycount) by 1,
- performs any BGP peer oscillation damping, and
- transitions to Idle state.
Established State:
In the Established state BGP can exchange UPDATE,
NOTFICATION, and KEEPALIVE messages with its peer.
If the local system receives an UPDATE message [Event26],
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the local system will:
- process the update packet
- restarts its Hold timer, if the negotiated Hold Time
value is non-zero, and
- remain in the Established state.
If the local system receives a NOTIFICATION message
[Event23 or Event24] or a disconnect [Event17] from the
underlying TCP, it:
- sets the appropriate error code in MIB reason code,
- if any BGP routes exist, delete all BGP routes,
- resets the connect retry timer (sets to zero),
- releases all the BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCnt (connect retry count)
by 1, and
- goes to the Idle state.
If the local system receives a Keepalive message
[Event 25], the local system will:
- restarts its Hold Timer, if the negotiated Hold Time
value is non-zero, and
- remain in the Established state.
If the local system receives an UPDATE message, and the
Update message error handling procedure (see Section 6.3)
detects an error [Event27], the local system:
- sends a NOTIFICATION message with Update error,
- resets the connect retry timer (sets to zero),
- drops the TCP connection,
- releases all BGP resources,
- increments the ConnectRetryCnt (connect retry count)
by 1,
- performs any BGP peer oscillation damping,
- and goes to Idle state.
Any start event (Event 1, 3-6) is ignored in the
Established state.
In response to a manual stop event (initiated by an
operator)[Event2], the local sytem:
- sets the Administrative stop in MIB reason code,
- sends the NOTIFICATION message with Cease,
- if BGP routes exist, delete the BGP routes,
- release BGP resources,
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- drops TCP connection,
- sets ConnectRetryCnt (connect retry count)
to zero (0),
- resets connect retry timer to zero (0), and
- transitions to the Idle.
In response to an automatic stop event initiated by the
system (automatic) [Event7], the local system:
- sets Administrative Stop in MIB Reason code,
- sends a NOTIFICATION with Cease,
- resets the connect retry timer (sets to zero)
- deletes all routes associated with bgp connection,
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCnt (connect retry count)
by 1,
- performs any BGP peer oscillation damping, and
- transitions to the idle state.
An example automatic stop event is exceeding the number of
prefixes for a given peer and the local system
automatically disconnecting the peer.
If the Hold timer expires [Event10], the local system:
- sends a NOTIFICATION message with Error Code Hold
Timer Expired,
- resets the connect retry timer (sets to zero),
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCnt (connect retry count)
by 1,
- performs any BGP peer oscillation damping,
- and goes to Idle state.
If the KeepAlive timer expires [Event11], the local system
sends a KEEPALIVE message, it restarts its KeepAlive timer,
unless the negotiated Hold Time value is zero.
Each time time the local system sends a KEEPALIVE or UPDATE
message, it restarts its KeepAlive timer, unless the
negotiated Hold Time value is zero.
A TCP connection indication [Event 13] received
for a valid port will cause the 2nd connection to be
tracked. A TCP connection indications for
invalid port [Event 14], will be ignored.
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In response to a TCP connection succeeds [Event 15
or Event 16], the 2nd connection shall be tracked until
it sends an OPEN message.
If a valid Open message [Event 18] is received, it will be
checked to see if it collides (section 6.8) with any other
session. If the BGP implementation determines that this
connection needs to be terminated, it will process an Call
Collision dump event[Event 22]. If this session needs to be
terminated, the connection will be terminated by:
- send a NOTIFICATION with a CEASE
- deletes all routes associated with connection,
- resets the connect retry timer,
- if any BGP routes, delete the routes,
- release all BGP resources,
- drops the TCP connection,
- increments ConnectRetryCnt (connect retry count)
by 1,
- and performs any BGP peer oscillation damping,
- and enters the Idle state
In response to any other event [Events 8-9,12, 19-21] the
local system:
- sends a NOTIFICATION message with Error Code Finite
State Machine Error,
- deletes all routes associated with BGP connection,
- resets the connect retry timer (sets to zero)
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCnt (connect retry count)
by 1,
- performs any BGP peer oscillation damping, and
- transitions to Idle.
9. UPDATE Message Handling
An UPDATE message may be received only in the Established state.
When an UPDATE message is received, each field is checked for valid-
ity as specified in Section 6.3.
If an optional non-transitive attribute is unrecognized, it is qui-
etly ignored. If an optional transitive attribute is unrecognized,
the Partial bit (the third high-order bit) in the attribute flags
octet is set to 1, and the attribute is retained for propagation to
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other BGP speakers.
If an optional attribute is recognized, and has a valid value, then,
depending on the type of the optional attribute, it is processed
locally, retained, and updated, if necessary, for possible propaga-
tion to other BGP speakers.
The information carried by the AS_PATH attribute is checked for AS
loops. AS loop detection is done by scanning the full AS path (as
specified in the AS_PATH attribute), and checking that the autonomous
system number of the local system does not appear in the AS path. If
the autonomous system number appears in the AS path the route may be
stored in the Adj-RIB-In, but unless the router is configured to
accept routes with its own autonomous system in the AS path, the
route shall not be passed to the BGP Decision Process. Operations of
a router that is configured to accept routes with its own autonomous
system number in the AS path are outside the scope of this document.
If the UPDATE message contains a non-empty WITHDRAWN ROUTES field,
the previously advertised routes whose destinations (expressed as IP
prefixes) contained in this field shall be removed from the Adj-RIB-
In. This BGP speaker shall run its Decision Process since the previ-
ously advertised route is no longer available for use.
If the UPDATE message contains a feasible route, the Adj-RIB-In will
be updated with this route as follows: if the NLRI of the new route
is identical to the one of the route currently stored in the Adj-RIB-
In, then the new route shall replace the older route in the Adj-RIB-
In, thus implicitly withdrawing the older route from service. Other-
wise, if the Adj-RIB-In has no route with NLRI identical to the new
route, the new route shall be placed in the Adj-RIB-In.
Once the BGP speaker updates the Adj-RIB-In, the speaker shall run
its Decision Process.
9.1 Decision Process
The Decision Process selects routes for subsequent advertisement by
applying the policies in the local Policy Information Base (PIB) to
the routes stored in its Adj-RIBs-In. The output of the Decision Pro-
cess is the set of routes that will be advertised to all peers; the
selected routes will be stored in the local speaker's Adj-RIB-Out.
The selection process is formalized by defining a function that takes
the attribute of a given route as an argument and returns either (a)
a non-negative integer denoting the degree of preference for the
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route, or (b) a value denoting that this route is ineligible to be
installed in LocRib and will be excluded from the next phase of route
selection.
The function that calculates the degree of preference for a given
route shall not use as its inputs any of the following: the existence
of other routes, the non-existence of other routes, or the path
attributes of other routes. Route selection then consists of individ-
ual application of the degree of preference function to each feasible
route, followed by the choice of the one with the highest degree of
preference.
The Decision Process operates on routes contained in the Adj-RIB-In,
and is responsible for:
- selection of routes to be used locally by the speaker
- selection of routes to be advertised to other BGP peers
- route aggregation and route information reduction
The Decision Process takes place in three distinct phases, each trig-
gered by a different event:
a) Phase 1 is responsible for calculating the degree of preference
for each route received from a peer.
b) Phase 2 is invoked on completion of phase 1. It is responsible
for choosing the best route out of all those available for each
distinct destination, and for installing each chosen route into
the Loc-RIB.
c) Phase 3 is invoked after the Loc-RIB has been modified. It is
responsible for disseminating routes in the Loc-RIB to each peer,
according to the policies contained in the PIB. Route aggregation
and information reduction can optionally be performed within this
phase.
9.1.1 Phase 1: Calculation of Degree of Preference
The Phase 1 decision function shall be invoked whenever the local BGP
speaker receives from a peer an UPDATE message that advertises a new
route, a replacement route, or withdrawn routes.
The Phase 1 decision function is a separate process which completes
when it has no further work to do.
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The Phase 1 decision function shall lock an Adj-RIB-In prior to oper-
ating on any route contained within it, and shall unlock it after
operating on all new or unfeasible routes contained within it.
For each newly received or replacement feasible route, the local BGP
speaker shall determine a degree of preference as follows:
If the route is learned from an internal peer, either the value of
the LOCAL_PREF attribute shall be taken as the degree of prefer-
ence, or the local system may compute the degree of preference of
the route based on preconfigured policy information. Note that the
latter (computing the degree of preference based on preconfigured
policy information) may result in formation of persistent routing
loops.
If the route is learned from an external peer, then the local BGP
speaker computes the degree of preference based on preconfigured
policy information. If the return value indicates that the route
is ineligible, the route may not serve as an input to the next
phase of route selection; otherwise the return value is used as
the LOCAL_PREF value in any IBGP readvertisement.
The exact nature of this policy information and the computation
involved is a local matter.
9.1.2 Phase 2: Route Selection
The Phase 2 decision function shall be invoked on completion of Phase
1. The Phase 2 function is a separate process which completes when it
has no further work to do. The Phase 2 process shall consider all
routes that are eligible in the Adj-RIBs-In.
The Phase 2 decision function shall be blocked from running while the
Phase 3 decision function is in process. The Phase 2 function shall
lock all Adj-RIBs-In prior to commencing its function, and shall
unlock them on completion.
If the NEXT_HOP attribute of a BGP route depicts an address that is
not resolvable, or it would become unresolvable if the route was
installed in the routing table the BGP route should be excluded from
the Phase 2 decision function.
It is critical that routers within an AS do not make conflicting
decisions regarding route selection that would cause forwarding loops
to occur.
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For each set of destinations for which a feasible route exists in the
Adj-RIBs-In, the local BGP speaker shall identify the route that has:
a) the highest degree of preference of any route to the same set
of destinations, or
b) is the only route to that destination, or
c) is selected as a result of the Phase 2 tie breaking rules spec-
ified in 9.1.2.2.
The local speaker SHALL then install that route in the Loc-RIB,
replacing any route to the same destination that is currently being
held in the Loc-RIB. When the new BGP route is installed in the Rout-
ing Table, care must be taken to ensure that existing routes to the
same destination that are now considered invalid are removed from the
Routing Table. Whether or not the new BGP route replaces an existing
non-BGP route in the Routing Table depends on the policy configured
on the BGP speaker.
The local speaker MUST determine the immediate next-hop address from
the NEXT_HOP attribute of the selected route (see section 5.1.3). If
either the immediate next hop or the IGP cost to the NEXT_HOP (where
the NEXT_HOP is resolved through an IGP route) changes, Phase 2:
Route Selection should be performed again.
Notice that even though BGP routes do not have to be installed in the
Routing Table with the immediate next hop(s), implementations must
take care that before any packets are forwarded along a BGP route,
its associated NEXT_HOP address is resolved to the immediate
(directly connected) next-hop address and this address (or multiple
addresses) is finally used for actual packet forwarding.
Unresolvable routes SHALL be removed from the Loc-RIB and the routing
table. However, corresponding unresolvable routes SHOULD be kept in
the Adj-RIBs-In (in case they become resolvable).
9.1.2.1 Route Resolvability Condition
As indicated in Section 9.1.2, BGP routers should exclude unresolv-
able routes from the Phase 2 decision. This ensures that only valid
routes are installed in Loc-RIB and the Routing Table.
The route resolvability condition is defined as follows.
1. A route Rte1, referencing only the intermediate network
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address, is considered resolvable if the Routing Table contains at
least one resolvable route Rte2 that matches Rte1's intermediate
network address and is not recursively resolved (directly or indi-
rectly) through Rte1. If multiple matching routes are available,
only the longest matching route should be considered.
2. Routes referencing interfaces (with or without intermediate
addresses) are considered resolvable if the state of the refer-
enced interface is up and IP processing is enabled on this inter-
face.
BGP routes do not refer to interfaces, but can be resolved through
the routes in the Routing Table that can be of both types (those that
specify interfaces or those that do not). IGP routes and routes to
directly connected networks are expected to specify the outbound
interface. Static routes can specify the outbound interface, or the
intermediate address, or both.
Note that a BGP route is considered unresolvable not only in situa-
tions where the router's Routing Table contains no route matching the
BGP route's NEXT_HOP. Mutually recursive routes (routes resolving
each other or themselves), also fail the resolvability check.
It is also important that implementations do not consider feasible
routes that would become unresolvable if they were installed in the
Routing Table even if their NEXT_HOPs are resolvable using the cur-
rent contents of the Routing Table (an example of such routes would
be mutually recursive routes). This check ensures that a BGP speaker
does not install in the Routing Table routes that will be removed and
not used by the speaker. Therefore, in addition to local Routing
Table stability, this check also improves behavior of the protocol in
the network.
Whenever a BGP speaker identifies a route that fails the resolvabil-
ity check because of mutual recursion, an error message should be
logged.
9.1.2.2 Breaking Ties (Phase 2)
In its Adj-RIBs-In a BGP speaker may have several routes to the same
destination that have the same degree of preference. The local
speaker can select only one of these routes for inclusion in the
associated Loc-RIB. The local speaker considers all routes with the
same degrees of preference, both those received from internal peers,
and those received from external peers.
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The following tie-breaking procedure assumes that for each candidate
route all the BGP speakers within an autonomous system can ascertain
the cost of a path (interior distance) to the address depicted by the
NEXT_HOP attribute of the route, and follow the same route selection
algorithm.
The tie-breaking algorithm begins by considering all equally prefer-
able routes to the same destination, and then selects routes to be
removed from consideration. The algorithm terminates as soon as only
one route remains in consideration. The criteria must be applied in
the order specified.
Several of the criteria are described using pseudo-code. Note that
the pseudo-code shown was chosen for clarity, not efficiency. It is
not intended to specify any particular implementation. BGP implemen-
tations MAY use any algorithm which produces the same results as
those described here.
a) Remove from consideration all routes which are not tied for
having the smallest number of AS numbers present in their AS_PATH
attributes. Note, that when counting this number, an AS_SET counts
as 1, no matter how many ASs are in the set.
b) Remove from consideration all routes which are not tied for
having the lowest Origin number in their Origin attribute.
c) Remove from consideration routes with less-preferred
MULTI_EXIT_DISC attributes. MULTI_EXIT_DISC is only comparable
between routes learned from the same neighboring AS. Routes which
do not have the MULTI_EXIT_DISC attribute are considered to have
the lowest possible MULTI_EXIT_DISC value.
This is also described in the following procedure:
for m = all routes still under consideration
for n = all routes still under consideration
if (neighborAS(m) == neighborAS(n)) and (MED(n) < MED(m))
remove route m from consideration
In the pseudo-code above, MED(n) is a function which returns the
value of route n's MULTI_EXIT_DISC attribute. If route n has no
MULTI_EXIT_DISC attribute, the function returns the lowest possi-
ble MULTI_EXIT_DISC value, i.e. 0.
Similarly, neighborAS(n) is a function which returns the neighbor
AS from which the route was received. If the route is learned via
IBGP, and the other IBGP speaker didn't originate the route, it is
the neighbor AS from which the other IBGP speaker learned the
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route. If the route is learned via IBGP, and the other IBGP
speaker originated the route, it is the local AS.
If a MULTI_EXIT_DISC attribute is removed before re-advertising a
route into IBGP, the MULTI_EXIT_DISC attribute may only be consid-
ered in the comparison of EBGP learned routes, then removed, then
the remaining EBGP learned route may be compared to the remaining
IBGP learned routes, without considering the MULTI_EXIT_DISC
attribute for those EBGP learned routes whose MULTI_EXIT_DISC will
be dropped before advertising to IBGP. Including the
MULTI_EXIT_DISC of an EBGP learned route in the comparison with an
IBGP learned route, then dropping the MULTI_EXIT_DISC and adver-
tising the route has been proven to cause route loops.
d) If at least one of the candidate routes was received from an
external peer in a neighboring autonomous system, remove from con-
sideration all routes which were received from internal peers.
e) Remove from consideration any routes with less-preferred inte-
rior cost. The interior cost of a route is determined by calcu-
lating the metric to the NEXT_HOP for the route using the Routing
Table. If the NEXT_HOP hop for a route is reachable, but no cost
can be determined, then this step should be skipped (equivalently,
consider all routes to have equal costs).
This is also described in the following procedure.
for m = all routes still under consideration
for n = all routes in still under consideration
if (cost(n) is better than cost(m))
remove m from consideration
In the pseudo-code above, cost(n) is a function which returns the
cost of the path (interior distance) to the address given in the
NEXT_HOP attribute of the route.
f) Remove from consideration all routes other than the route that
was advertised by the BGP speaker whose BGP Identifier has the
lowest value.
g) Prefer the route received from the lowest neighbor address.
9.1.3 Phase 3: Route Dissemination
The Phase 3 decision function shall be invoked on completion of Phase
2, or when any of the following events occur:
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a) when routes in the Loc-RIB to local destinations have changed
b) when locally generated routes learned by means outside of BGP
have changed
c) when a new BGP speaker - BGP speaker connection has been estab-
lished
The Phase 3 function is a separate process which completes when it
has no further work to do. The Phase 3 Routing Decision function
shall be blocked from running while the Phase 2 decision function is
in process.
All routes in the Loc-RIB shall be processed into Adj-RIBs-Out
according to configured policy. This policy may exclude a route in
the Loc-RIB from being installed in a particular Adj-RIB-Out. A
route shall not be installed in the Adj-Rib-Out unless the destina-
tion and NEXT_HOP described by this route may be forwarded appropri-
ately by the Routing Table. If a route in Loc-RIB is excluded from a
particular Adj-RIB-Out the previously advertised route in that Adj-
RIB-Out must be withdrawn from service by means of an UPDATE message
(see 9.2).
Route aggregation and information reduction techniques (see 9.2.2.1)
may optionally be applied.
Any local policy which results in routes being added to an Adj-RIB-
Out without also being added to the local BGP speaker's forwarding
table, is outside the scope of this document.
When the updating of the Adj-RIBs-Out and the Routing Table is com-
plete, the local BGP speaker shall run the Update-Send process of
9.2.
9.1.4 Overlapping Routes
A BGP speaker may transmit routes with overlapping Network Layer
Reachability Information (NLRI) to another BGP speaker. NLRI overlap
occurs when a set of destinations are identified in non-matching mul-
tiple routes. Since BGP encodes NLRI using IP prefixes, overlap will
always exhibit subset relationships. A route describing a smaller
set of destinations (a longer prefix) is said to be more specific
than a route describing a larger set of destinations (a shorter pre-
fix); similarly, a route describing a larger set of destinations is
said to be less specific than a route describing a smaller set of
destinations.
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The precedence relationship effectively decomposes less specific
routes into two parts:
- a set of destinations described only by the less specific route,
and
- a set of destinations described by the overlap of the less spe-
cific and the more specific routes
When overlapping routes are present in the same Adj-RIB-In, the more
specific route shall take precedence, in order from more specific to
least specific.
The set of destinations described by the overlap represents a portion
of the less specific route that is feasible, but is not currently in
use. If a more specific route is later withdrawn, the set of desti-
nations described by the overlap will still be reachable using the
less specific route.
If a BGP speaker receives overlapping routes, the Decision Process
MUST consider both routes based on the configured acceptance policy.
If both a less and a more specific route are accepted, then the Deci-
sion Process MUST either install both the less and the more specific
routes or it MUST aggregate the two routes and install the aggregated
route, provided that both routes have the same value of the NEXT_HOP
attribute.
If a BGP speaker chooses to aggregate, then it SHOULD either include
all AS used to form the aggreagate in an AS_SET or add the
ATOMIC_AGGREGATE attribute to the route. This attribute is now pri-
marily informational. With the elimination of IP routing protocols
that do not support classless routing and the elimination of router
and host implementations that do not support classless routing, there
is no longer a need to deaggregate. Routes SHOULD NOT be de-aggre-
gated. A route that carries ATOMIC_AGGREGATE attribute in particular
MUST NOT be de-aggregated. That is, the NLRI of this route can not be
made more specific. Forwarding along such a route does not guarantee
that IP packets will actually traverse only ASs listed in the AS_PATH
attribute of the route.
9.2 Update-Send Process
The Update-Send process is responsible for advertising UPDATE mes-
sages to all peers. For example, it distributes the routes chosen by
the Decision Process to other BGP speakers which may be located in
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either the same autonomous system or a neighboring autonomous system.
When a BGP speaker receives an UPDATE message from an internal peer,
the receiving BGP speaker shall not re-distribute the routing infor-
mation contained in that UPDATE message to other internal peers,
unless the speaker acts as a BGP Route Reflector [RFC2796].
As part of Phase 3 of the route selection process, the BGP speaker
has updated its Adj-RIBs-Out. All newly installed routes and all
newly unfeasible routes for which there is no replacement route shall
be advertised to its peers by means of an UPDATE message.
A BGP speaker should not advertise a given feasible BGP route from
its Adj-RIB-Out if it would produce an UPDATE message containing the
same BGP route as was previously advertised.
Any routes in the Loc-RIB marked as unfeasible shall be removed.
Changes to the reachable destinations within its own autonomous sys-
tem shall also be advertised in an UPDATE message.
If due to the limits on the maximum size of an UPDATE message (see
Section 4) a single route doesn't fit into the message, the BGP
speaker MUST not advertise the route to its peers and MAY choose to
log an error locally.
9.2.1 Controlling Routing Traffic Overhead
The BGP protocol constrains the amount of routing traffic (that is,
UPDATE messages) in order to limit both the link bandwidth needed to
advertise UPDATE messages and the processing power needed by the
Decision Process to digest the information contained in the UPDATE
messages.
9.2.1.1 Frequency of Route Advertisement
The parameter MinRouteAdvertisementInterval determines the minimum
amount of time that must elapse between advertisement and/or with-
drawal of routes to a particular destination by a BGP speaker to a
peer. This rate limiting procedure applies on a per-destination
basis, although the value of MinRouteAdvertisementInterval is set on
a per BGP peer basis.
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Two UPDATE messages sent by a BGP speaker to a peer that advertise
feasible routes and/or withdrawal of unfeasible routes to some common
set of destinations MUST be separated by at least MinRouteAdvertise-
mentInterval. Clearly, this can only be achieved precisely by keeping
a separate timer for each common set of destinations. This would be
unwarranted overhead. Any technique which ensures that the interval
between two UPDATE messages sent from a BGP speaker to a peer that
advertise feasible routes and/or withdrawal of unfeasible routes to
some common set of destinations will be at least MinRouteAdvertise-
mentInterval, and will also ensure a constant upper bound on the
interval is acceptable.
Since fast convergence is needed within an autonomous system, either
(a) the MinRouteAdvertisementInterval used for internal peers SHOULD
be shorter than the MinRouteAdvertisementInterval used for external
peers, or (b) the procedure describe in this section SHOULD NOT apply
for routes sent to internal peers.
This procedure does not limit the rate of route selection, but only
the rate of route advertisement. If new routes are selected multiple
times while awaiting the expiration of MinRouteAdvertisementInterval,
the last route selected SHALL be advertised at the end of MinRouteAd-
vertisementInterval.
9.2.1.2 Frequency of Route Origination
The parameter MinASOriginationInterval determines the minimum amount
of time that must elapse between successive advertisements of UPDATE
messages that report changes within the advertising BGP speaker's own
autonomous systems.
9.2.2 Efficient Organization of Routing Information
Having selected the routing information which it will advertise, a
BGP speaker may avail itself of several methods to organize this
information in an efficient manner.
9.2.2.1 Information Reduction
Information reduction may imply a reduction in granularity of policy
control - after information is collapsed, the same policies will
apply to all destinations and paths in the equivalence class.
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The Decision Process may optionally reduce the amount of information
that it will place in the Adj-RIBs-Out by any of the following meth-
ods:
a) Network Layer Reachability Information (NLRI):
Destination IP addresses can be represented as IP address pre-
fixes. In cases where there is a correspondence between the
address structure and the systems under control of an autonomous
system administrator, it will be possible to reduce the size of
the NLRI carried in the UPDATE messages.
b) AS_PATHs:
AS path information can be represented as ordered AS_SEQUENCEs or
unordered AS_SETs. AS_SETs are used in the route aggregation algo-
rithm described in 9.2.2.2. They reduce the size of the AS_PATH
information by listing each AS number only once, regardless of how
many times it may have appeared in multiple AS_PATHs that were
aggregated.
An AS_SET implies that the destinations listed in the NLRI can be
reached through paths that traverse at least some of the con-
stituent autonomous systems. AS_SETs provide sufficient informa-
tion to avoid routing information looping; however their use may
prune potentially feasible paths, since such paths are no longer
listed individually as in the form of AS_SEQUENCEs. In practice
this is not likely to be a problem, since once an IP packet
arrives at the edge of a group of autonomous systems, the BGP
speaker at that point is likely to have more detailed path infor-
mation and can distinguish individual paths to destinations.
9.2.2.2 Aggregating Routing Information
Aggregation is the process of combining the characteristics of sev-
eral different routes in such a way that a single route can be adver-
tised. Aggregation can occur as part of the decision process to
reduce the amount of routing information that will be placed in the
Adj-RIBs-Out.
Aggregation reduces the amount of information that a BGP speaker must
store and exchange with other BGP speakers. Routes can be aggregated
by applying the following procedure separately to path attributes of
like type and to the Network Layer Reachability Information.
Routes that have different MULTI_EXIT_DISC attribute SHALL NOT be
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aggregated.
Path attributes that have different type codes can not be aggregated
together. Path attributes of the same type code may be aggregated,
according to the following rules:
NEXT_HOP:
When aggregating routes that have different NEXT_HOP attribute,
the NEXT_HOP attribute of the aggregated route SHALL identify
an interface on the router that performs the aggregation.
ORIGIN attribute:
If at least one route among routes that are aggregated has ORI-
GIN with the value INCOMPLETE, then the aggregated route must
have the ORIGIN attribute with the value INCOMPLETE. Other-
wise, if at least one route among routes that are aggregated
has ORIGIN with the value EGP, then the aggregated route must
have the origin attribute with the value EGP. In all other case
the value of the ORIGIN attribute of the aggregated route is
IGP.
AS_PATH attribute:
If routes to be aggregated have identical AS_PATH attributes,
then the aggregated route has the same AS_PATH attribute as
each individual route.
For the purpose of aggregating AS_PATH attributes we model each
AS within the AS_PATH attribute as a tuple <type, value>, where
"type" identifies a type of the path segment the AS belongs to
(e.g. AS_SEQUENCE, AS_SET), and "value" is the AS number. If
the routes to be aggregated have different AS_PATH attributes,
then the aggregated AS_PATH attribute shall satisfy all of the
following conditions:
- all tuples of type AS_SEQUENCE in the aggregated AS_PATH
shall appear in all of the AS_PATH in the initial set of
routes to be aggregated.
- all tuples of type AS_SET in the aggregated AS_PATH shall
appear in at least one of the AS_PATH in the initial set
(they may appear as either AS_SET or AS_SEQUENCE types).
- for any tuple X of type AS_SEQUENCE in the aggregated
AS_PATH which precedes tuple Y in the aggregated AS_PATH, X
precedes Y in each AS_PATH in the initial set which contains
Y, regardless of the type of Y.
- No tuple of type AS_SET with the same value shall appear
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more than once in the aggregated AS_PATH.
- Multiple tuples of type AS_SEQUENCE with the same value
may appear in the aggregated AS_PATH only when adjacent to
another tuple of the same type and value.
An implementation may choose any algorithm which conforms to
these rules. At a minimum a conformant implementation shall be
able to perform the following algorithm that meets all of the
above conditions:
- determine the longest leading sequence of tuples (as
defined above) common to all the AS_PATH attributes of the
routes to be aggregated. Make this sequence the leading
sequence of the aggregated AS_PATH attribute.
- set the type of the rest of the tuples from the AS_PATH
attributes of the routes to be aggregated to AS_SET, and
append them to the aggregated AS_PATH attribute.
- if the aggregated AS_PATH has more than one tuple with the
same value (regardless of tuple's type), eliminate all, but
one such tuple by deleting tuples of the type AS_SET from
the aggregated AS_PATH attribute.
- for each pair of adjacent tuples in the aggregated
AS_PATH, if both tuples have the same type, merge them
together, as long as doing so will not cause a segment with
length greater than 255 to be generated.
Appendix F, section F.6 presents another algorithm that satis-
fies the conditions and allows for more complex policy configu-
rations.
ATOMIC_AGGREGATE:
If at least one of the routes to be aggregated has
ATOMIC_AGGREGATE path attribute, then the aggregated route
shall have this attribute as well.
AGGREGATOR:
All AGGREGATOR attributes of all routes to be aggregated should
be ignored. The BGP speaker performing the route aggregation
may attach a new AGGREGATOR attribute (see Section 5.1.7).
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9.3 Route Selection Criteria
Generally speaking, additional rules for comparing routes among sev-
eral alternatives are outside the scope of this document. There are
two exceptions:
- If the local AS appears in the AS path of the new route being
considered, then that new route can not be viewed as better than
any other route (provided that the speaker is configured to accept
such routes). If such a route were ever used, a routing loop could
result (see Section 6.3).
- In order to achieve successful distributed operation, only
routes with a likelihood of stability can be chosen. Thus, an AS
must avoid using unstable routes, and it must not make rapid spon-
taneous changes to its choice of route. Quantifying the terms
"unstable" and "rapid" in the previous sentence will require expe-
rience, but the principle is clear.
Care must be taken to ensure that BGP speakers in the same AS do not
make inconsistent decisions.
9.4 Originating BGP routes
A BGP speaker may originate BGP routes by injecting routing informa-
tion acquired by some other means (e.g. via an IGP) into BGP. A BGP
speaker that originates BGP routes shall assign the degree of prefer-
ence to these routes by passing them through the Decision Process
(see Section 9.1). These routes may also be distributed to other BGP
speakers within the local AS as part of the update process (see Sec-
tion 9.2). The decision whether to distribute non-BGP acquired routes
within an AS via BGP or not depends on the environment within the AS
(e.g. type of IGP) and should be controlled via configuration.
10 BGP Timers
BGP employs five timers: ConnectRetry (see Section 8), Hold Time (see
Section 4.2), KeepAlive (see Section 8), MinASOriginationInterval
(see Section 9.2.1.2), and MinRouteAdvertisementInterval (see Section
9.2.1.1).
The suggested default value for the ConnectRetry timer is 120 sec-
onds.
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The suggested default value for the Hold Time is 90 seconds.
The suggested default value for the KeepAlive timer is 1/3 of the
Hold Time.
The suggested default value for the MinASOriginationInterval is 15
seconds.
The suggested default value for the MinRouteAdvertisementInterval is
30 seconds.
An implementation of BGP MUST allow the Hold Time timer to be config-
urable on a per peer basis, and MAY allow the other timers to be con-
figurable.
To minimize the likelihood that the distribution of BGP messages by a
given BGP speaker will contain peaks, jitter should be applied to the
timers associated with MinASOriginationInterval, KeepAlive, Min-
RouteAdvertisementInterval, and ConnectRetry. A given BGP speaker may
apply the same jitter to each of these quantities regardless of the
destinations to which the updates are being sent; that is, jitter
need not be configured on a "per peer" basis.
The suggested default amount of jitter shall be determined by multi-
plying the base value of the appropriate timer by a random factor
which is uniformly distributed in the range from 0.75 to 1.0. A new
random value should be picked each time the timer is set. The range
of the jitter random value MAY be configurable.
Appendix A. Comparison with RFC1771
There are numerous editorial changes (too many to list here).
The following list the technical changes:
Changes to reflect the usages of such features as TCP MD5
[RFC2385], BGP Route Reflectors [RFC2796], BGP Confederations
[RFC3065], and BGP Route Refresh [RFC2918].
Clarification on the use of the BGP Identifier in the AGGREGATOR
attribute.
Procedures for imposing an upper bound on the number of prefixes
that a BGP speaker would accept from a peer.
The ability of a BGP speaker to include more than one instance of
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its own AS in the AS_PATH attribute for the purpose of inter-AS
traffic engineering.
Clarifications on the various types of NEXT_HOPs.
Clarifications to the use of the ATOMIC_AGGREGATE attribute.
The relationship between the immediate next hop, and the next hop
as specified in the NEXT_HOP path attribute.
Clarifications on the tie-breaking procedures.
Clarifications on the frequency of route advertisements.
Optional Parameter Type 1 (Authentication Information) has been
deprecated.
UPDATE Message Error subcode 7 (AS Routing Loop) has been depre-
cated.
Use of the Marker field for authentication has been deprecated.
Appendix B. Comparison with RFC1267
All the changes listed in Appendix A, plus the following.
BGP-4 is capable of operating in an environment where a set of reach-
able destinations may be expressed via a single IP prefix. The con-
cept of network classes, or subnetting is foreign to BGP-4. To
accommodate these capabilities BGP-4 changes semantics and encoding
associated with the AS_PATH attribute. New text has been added to
define semantics associated with IP prefixes. These abilities allow
BGP-4 to support the proposed supernetting scheme [9].
To simplify configuration this version introduces a new attribute,
LOCAL_PREF, that facilitates route selection procedures.
The INTER_AS_METRIC attribute has been renamed to be MULTI_EXIT_DISC.
A new attribute, ATOMIC_AGGREGATE, has been introduced to insure that
certain aggregates are not de-aggregated. Another new attribute,
AGGREGATOR, can be added to aggregate routes in order to advertise
which AS and which BGP speaker within that AS caused the aggregation.
To insure that Hold Timers are symmetric, the Hold Time is now nego-
tiated on a per-connection basis. Hold Times of zero are now sup-
ported.
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Appendix C. Comparison with RFC 1163
All of the changes listed in Appendices A and B, plus the following.
To detect and recover from BGP connection collision, a new field (BGP
Identifier) has been added to the OPEN message. New text (Section
6.8) has been added to specify the procedure for detecting and recov-
ering from collision.
The new document no longer restricts the border router that is passed
in the NEXT_HOP path attribute to be part of the same Autonomous Sys-
tem as the BGP Speaker.
New document optimizes and simplifies the exchange of the information
about previously reachable routes.
Appendix D. Comparison with RFC 1105
All of the changes listed in Appendices A, B and C, plus the follow-
ing.
Minor changes to the RFC1105 Finite State Machine were necessary to
accommodate the TCP user interface provided by 4.3 BSD.
The notion of Up/Down/Horizontal relations present in RFC1105 has
been removed from the protocol.
The changes in the message format from RFC1105 are as follows:
1. The Hold Time field has been removed from the BGP header and
added to the OPEN message.
2. The version field has been removed from the BGP header and
added to the OPEN message.
3. The Link Type field has been removed from the OPEN message.
4. The OPEN CONFIRM message has been eliminated and replaced with
implicit confirmation provided by the KEEPALIVE message.
5. The format of the UPDATE message has been changed signifi-
cantly. New fields were added to the UPDATE message to support
multiple path attributes.
6. The Marker field has been expanded and its role broadened to
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support authentication.
Note that quite often BGP, as specified in RFC 1105, is referred
to as BGP-1, BGP, as specified in RFC 1163, is referred to as
BGP-2, BGP, as specified in RFC1267 is referred to as BGP-3, and
BGP, as specified in this document is referred to as BGP-4.
Appendix E. TCP options that may be used with BGP
If a local system TCP user interface supports TCP PUSH function, then
each BGP message should be transmitted with PUSH flag set. Setting
PUSH flag forces BGP messages to be transmitted promptly to the
receiver.
If a local system TCP user interface supports setting precedence for
TCP connection, then TCP connection used by BGP should be opened with
precedence set to Internetwork Control (110) value (see also
[RFC791]).
A local system may protect its BGP connections by using the TCP MD5
Signature Option [RFC2385].
Appendix F. Implementation Recommendations
This section presents some implementation recommendations.
Appendix F.1 Multiple Networks Per Message
The BGP protocol allows for multiple address prefixes with the same
path attributes to be specified in one message. Making use of this
capability is highly recommended. With one address prefix per message
there is a substantial increase in overhead in the receiver. Not only
does the system overhead increase due to the reception of multiple
messages, but the overhead of scanning the routing table for updates
to BGP peers and other routing protocols (and sending the associated
messages) is incurred multiple times as well.
One method of building messages containing many address prefixes per
a path attribute set from a routing table that is not organized on a
per path attribute set basis is to build many messages as the routing
table is scanned. As each address prefix is processed, a message for
the associated set of path attributes is allocated, if it does not
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exist, and the new address prefix is added to it. If such a message
exists, the new address prefix is just appended to it. If the message
lacks the space to hold the new address prefix, it is transmitted, a
new message is allocated, and the new address prefix is inserted into
the new message. When the entire routing table has been scanned, all
allocated messages are sent and their resources released. Maximum
compression is achieved when all the destinations covered by the
address prefixes share a common set of path attributes making it pos-
sible to send many address prefixes in one 4096-byte message.
When peering with a BGP implementation that does not compress multi-
ple address prefixes into one message, it may be necessary to take
steps to reduce the overhead from the flood of data received when a
peer is acquired or a significant network topology change occurs. One
method of doing this is to limit the rate of updates. This will elim-
inate the redundant scanning of the routing table to provide flash
updates for BGP peers and other routing protocols. A disadvantage of
this approach is that it increases the propagation latency of routing
information. By choosing a minimum flash update interval that is not
much greater than the time it takes to process the multiple messages
this latency should be minimized. A better method would be to read
all received messages before sending updates.
Appendix F.2 Reducing route flapping
To avoid excessive route flapping a BGP speaker which needs to with-
draw a destination and send an update about a more specific or less
specific route SHOULD combine them into the same UPDATE message.
Appendix F.3 Path attribute ordering
Implementations which combine update messages as described above in
6.1 may prefer to see all path attributes presented in a known order.
This permits them to quickly identify sets of attributes from differ-
ent update messages which are semantically identical. To facilitate
this, it is a useful optimization to order the path attributes
according to type code. This optimization is entirely optional.
Appendix F.4 AS_SET sorting
Another useful optimization that can be done to simplify this situa-
tion is to sort the AS numbers found in an AS_SET. This optimization
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is entirely optional.
Appendix F.5 Control over version negotiation
Since BGP-4 is capable of carrying aggregated routes which can not be
properly represented in BGP-3, an implementation which supports BGP-4
and another BGP version should provide the capability to only speak
BGP-4 on a per-peer basis.
Appendix F.6 Complex AS_PATH aggregation
An implementation which chooses to provide a path aggregation algo-
rithm which retains significant amounts of path information may wish
to use the following procedure:
For the purpose of aggregating AS_PATH attributes of two routes,
we model each AS as a tuple <type, value>, where "type" identifies
a type of the path segment the AS belongs to (e.g. AS_SEQUENCE,
AS_SET), and "value" is the AS number. Two ASs are said to be the
same if their corresponding <type, value> tuples are the same.
The algorithm to aggregate two AS_PATH attributes works as fol-
lows:
a) Identify the same ASs (as defined above) within each AS_PATH
attribute that are in the same relative order within both
AS_PATH attributes. Two ASs, X and Y, are said to be in the
same order if either:
- X precedes Y in both AS_PATH attributes, or - Y precedes X
in both AS_PATH attributes.
b) The aggregated AS_PATH attribute consists of ASs identified
in (a) in exactly the same order as they appear in the AS_PATH
attributes to be aggregated. If two consecutive ASs identified
in (a) do not immediately follow each other in both of the
AS_PATH attributes to be aggregated, then the intervening ASs
(ASs that are between the two consecutive ASs that are the
same) in both attributes are combined into an AS_SET path seg-
ment that consists of the intervening ASs from both AS_PATH
attributes; this segment is then placed in between the two con-
secutive ASs identified in (a) of the aggregated attribute. If
two consecutive ASs identified in (a) immediately follow each
other in one attribute, but do not follow in another, then the
intervening ASs of the latter are combined into an AS_SET path
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segment; this segment is then placed in between the two consec-
utive ASs identified in (a) of the aggregated attribute.
c) For each pair of adjacent tuples in the aggregated AS_PATH,
if both tuples have the same type, merge them together, as long
as doing so will not cause a segment with length greater than
255 to be generated.
If as a result of the above procedure a given AS number appears
more than once within the aggregated AS_PATH attribute, all, but
the last instance (rightmost occurrence) of that AS number should
be removed from the aggregated AS_PATH attribute.
Security Considerations
BGP supports the ability to authenticate BGP messages by using BGP
authentication. The authentication could be done on a per peer basis.
In addition, BGP supports the ability to authenticate its data stream
by using [RFC2385]. This authentication could be done on a per peer
basis. Finally, BGP could also use IPSec to authenticate its data
stream. Among the mechanisms mentioned in this paragraph, [RFC2385]
is the most widely deployed.
Normative References
[RFC793] Postel, J., "Transmission Control Protocol - DARPA Internet
Program Protocol Specification", RFC793, September 1981.
[RFC791] Postel, J., "Internet Protocol - DARPA Internet Program Pro-
tocol Specification", RFC791, September 1981.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Non-normative References
[RFC904] Mills, D., "Exterior Gateway Protocol Formal Specification",
RFC904, April 1984.
[RFC1092] Rekhter, Y., "EGP and Policy Based Routing in the New
NSFNET Backbone", RFC1092, February 1989.
[RFC1093] Braun, H-W., "The NSFNET Routing Architecture", RFC1093,
February 1989.
Expiration Date April 2003 [Page 79]
RFC DRAFT October 2002
[RFC1772] Rekhter, Y., and P. Gross, "Application of the Border Gate-
way Protocol in the Internet", RFC1772, March 1995.
[RFC1518] Rekhter, Y., Li, T., "An Architecture for IP Address Allo-
cation with CIDR", RFC 1518, September 1993.
[RFC1519] Fuller, V., Li, T., Yu, J., and Varadhan, K., ""Classless
Inter-Domain Routing (CIDR): an Address Assignment and Aggregation
Strategy", RFC1519, September 1993.
[RFC1997] R. Chandra, P. Traina, T. Li, "BGP Communities Attribute",
RFC 1997, August 1996.
[RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC2385, August 1998.
[RFC2439] C. Villamizar, R. Chandra, R. Govindan, "BGP Route Flap
Damping", RFC2439, November 1998.
[RFC2796] Bates, T., Chandra, R., Chen, E., "BGP Route Reflection -
An Alternative to Full Mesh IBGP", RFC2796, April 2000.
[RFC2842] R. Chandra, J. Scudder, "Capabilities Advertisement with
BGP-4", RFC2842.
[RFC2858] T. Bates, R. Chandra, D. Katz, Y. Rekhter, "Multiprotocol
Extensions for BGP-4", RFC2858.
[RFC2918] Chen, E., "Route Refresh Capability for BGP-4", RFC2918,
September 2000.
[RFC3065] Traina, P, McPherson, D., Scudder, J., "Autonomous System
Confederations for BGP", RFC3065, February 2001.
[IS10747] "Information Processing Systems - Telecommunications and
Information Exchange between Systems - Protocol for Exchange of
Inter-domain Routeing Information among Intermediate Systems to Sup-
port Forwarding of ISO 8473 PDUs", ISO/IEC IS10747, 1993
Editors' Addresses
Yakov Rekhter
Juniper Networks
email: yakov@juniper.net
Tony Li
Procket Networks, Inc.
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email: tli@procket.com
Susan Hares
NextHop Technologies, Inc.
email: skh@nexthop.com
Expiration Date April 2003 [Page 81]
