Network Working Group Tony Bates
Internet Draft Cisco Systems
Expiration Date: February 1999 Ravi Chandra
Cisco Systems
Dave Katz
Juniper Networks
Yakov Rekhter
Cisco Systems
Multiprotocol Extensions for BGP-4
draft-ietf-idr-bgp4-multiprotocol-v2-01.txt
1. Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
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2. Abstract
Currently BGP-4 [BGP-4] is capable of carrying routing information
only for IPv4 [IPv4]. This document defines extensions to BGP-4 to
enable it to carry routing information for multiple Network Layer
protocols (e.g., IPv6, IPX, etc...). The extensions are backward
compatible - a router that supports the extensions can interoperate
with a router that doesn't support the extensions.
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3. Overview
The only three pieces of information carried by BGP-4 that are IPv4
specific are (a) the NEXT_HOP attribute (expressed as an IPv4
address), (b) AGGREGATOR (contains an IPv4 address), and (c) NLRI
(expressed as IPv4 address prefixes). This document assumes that any
BGP speaker (including the one that supports multiprotocol
capabilities defined in this document) has to have an IPv4 address
(which will be used, among other things, in the AGGREGATOR
attribute). Therefore, to enable BGP-4 to support routing for
multiple Network Layer protocols the only two things that have to be
added to BGP-4 are (a) the ability to associate a particular Network
Layer protocol with the next hop information, and (b) the ability to
associated a particular Network Layer protocol with NLRI. To identify
individual Network Layer protocols this document uses Address Family,
as defined in [RFC1700].
One could further observe that the next hop information (the
information provided by the NEXT_HOP attribute) is meaningful (and
necessary) only in conjunction with the advertisements of reachable
destinations - in conjunction with the advertisements of unreachable
destinations (withdrawing routes from service) the next hop
information is meaningless. This suggests that the advertisement of
reachable destinations should be grouped with the advertisement of
the next hop to be used for these destinations, and that the
advertisement of reachable destinations should be segregated from the
advertisement of unreachable destinations.
To provide backward compatibility, as well as to simplify
introduction of the multiprotocol capabilities into BGP-4 this
document uses two new attributes, Multiprotocol Reachable NLRI
(MP_REACH_NLRI), and Multiprotocol Unreachable NLRI
(MP_UNREACH_NLRI). The first one (MP_REACH_NLRI) is used to carry the
set of reachable destinations together with the next hop information
to be used for forwarding to these destinations. The second one
(MP_UNREACH_NLRI) is used to carry the set of unreachable
destinations. Both of these attributes are optional and non-
transitive. This way a BGP speaker that doesn't support the
multiprotocol capabilities will just ignore the information carried
in these attributes, and will not pass it to other BGP speakers.
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4. Multiprotocol Reachable NLRI - MP_REACH_NLRI (Type Code 14):
This is an optional non-transitive attribute that can be used for the
following purposes:
(a) to advertise a feasible route to a peer
(b) to permit a router to advertise the Network Layer address of
the router that should be used as the next hop to the destinations
listed in the Network Layer Reachability Information field of the
MP_NLRI attribute.
(c) to allow a given router to report some or all of the
Subnetwork Points of Attachment (SNPAs) that exist within the
local system
The attribute is encoded as shown below:
+---------------------------------------------------------+
| Address Family Identifier (2 octets) |
+---------------------------------------------------------+
| Subsequent Address Family Identifier (1 octet) |
+---------------------------------------------------------+
| Length of Next Hop Network Address (1 octet) |
+---------------------------------------------------------+
| Network Address of Next Hop (variable) |
+---------------------------------------------------------+
| Number of SNPAs (1 octet) |
+---------------------------------------------------------+
| Length of first SNPA(1 octet) |
+---------------------------------------------------------+
| First SNPA (variable) |
+---------------------------------------------------------+
| Length of second SNPA (1 octet) |
+---------------------------------------------------------+
| Second SNPA (variable) |
+---------------------------------------------------------+
| ... |
+---------------------------------------------------------+
| Length of Last SNPA (1 octet) |
+---------------------------------------------------------+
| Last SNPA (variable) |
+---------------------------------------------------------+
| Network Layer Reachability Information (variable) |
+---------------------------------------------------------+
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The use and meaning of these fields are as follows:
Address Family Identifier:
This field carries the identity of the Network Layer protocol
associated with the Network Address that follows. Presently
defined values for this field are specified in RFC1700 (see the
Address Family Numbers section).
Subsequent Address Family Identifier:
This field provides additional information about the type of
the Network Layer Reachability Information carried in the
attribute.
Length of Next Hop Network Address:
A 1 octet field whose value expresses the length of the
"Network Address of Next Hop" field as measured in octets
Network Address of Next Hop:
A variable length field that contains the Network Address of
the next router on the path to the destination system
Number of SNPAs:
A 1 octet field which contains the number of distinct SNPAs to
be listed in the following fields. The value 0 may be used to
indicate that no SNPAs are listed in this attribute.
Length of Nth SNPA:
A 1 octet field whose value expresses the length of the "Nth
SNPA of Next Hop" field as measured in semi-octets
Nth SNPA of Next Hop:
A variable length field that contains an SNPA of the router
whose Network Address is contained in the "Network Address of
Next Hop" field. The field length is an integral number of
octets in length, namely the rounded-up integer value of one
half the SNPA length expressed in semi-octets; if the SNPA
contains an odd number of semi-octets, a value in this field
will be padded with a trailing all-zero semi-octet.
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Network Layer Reachability Information:
A variable length field that lists NLRI for the feasible routes
that are being advertised in this attribute. When the
Subsequent Address Family Identifier field is set to one of the
values defined in this document, each NLRI is encoded as
specified in the "NLRI encoding" section of this document.
The next hop information carried in the MP_REACH_NLRI path attribute
defines the Network Layer address of the border router that should be
used as the next hop to the destinations listed in the MP_NLRI
attribute in the UPDATE message. When advertising a MP_REACH_NLRI
attribute to an external peer, a router may use one of its own
interface addresses in the next hop component of the attribute,
provided the external peer to which the route is being advertised
shares a common subnet with the next hop address. This is known as a
"first party" next hop. A BGP speaker can advertise to an external
peer an interface of any internal peer router in the next hop
component, provided the external peer to which the route is being
advertised shares a common subnet with the next hop address. This is
known as a "third party" next hop information. A BGP speaker can
advertise any external peer router in the next hop component,
provided that the Network Layer address of this border router was
learned from an external peer, and the external peer to which the
route is being advertised shares a common subnet with the next hop
address. This is a second form of "third party" next hop
information.
Normally the next hop information 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 information to handle
imperfectly bridged media or for reasons of policy.
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.
When a BGP speaker advertises the route to an internal peer, the
advertising speaker should not modify the next hop information
associated with the route. When a BGP speaker receives the route via
an internal link, it may forward packets to the next hop address if
the address contained in the attribute is on a common subnet with the
local and remote BGP speakers.
An UPDATE message that carries the MP_REACH_NLRI must also carry the
ORIGIN and the AS_PATH attributes (both in EBGP and in IBGP
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exchanges). Moreover, in IBGP exchanges such a message must also
carry the LOCAL_PREF attribute. If such a message is received from an
external peer, the local system shall check whether the leftmost AS
in the AS_PATH attribute is equal to the autonomous system number of
the peer than sent the message. If that is not the case, the local
system shall send the NOTIFICATION message with Error Code UPDATE
Message Error, and the Error Subcode set to Malformed AS_PATH.
An UPDATE message that carries no NLRI, other than the one encoded in
the MP_REACH_NLRI attribute, should not carry the NEXT_HOP attribute.
If such a message contains the NEXT_HOP attribute, the BGP speaker
that receives the message should ignore this attribute.
5. Multiprotocol Unreachable NLRI - MP_UNREACH_NLRI (Type Code 15):
This is an optional non-transitive attribute that can be used for the
purpose of withdrawing multiple unfeasible routes from service.
The attribute is encoded as shown below:
+---------------------------------------------------------+
| Address Family Identifier (2 octets) |
+---------------------------------------------------------+
| Subsequent Address Family Identifier (1 octet) |
+---------------------------------------------------------+
| Withdrawn Routes (variable) |
+---------------------------------------------------------+
The use and the meaning of these fields are as follows:
Address Family Identifier:
This field carries the identity of the Network Layer protocol
associated with the NLRI that follows. Presently defined values
for this field are specified in RFC1700 (see the Address Family
Numbers section).
Subsequent Address Family Identifier:
This field provides additional information about the type of
the Network Layer Reachability Information carried in the
attribute.
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Withdrawn Routes:
A variable length field that lists NLRI for the routes that are
being withdrawn from service. When the Subsequent Address
Family Identifier field is set to one of the values defined in
this document, each NLRI is encoded as specified in the "NLRI
encoding" section of this document.
An UPDATE message that contains the MP_UNREACH_NLRI is not required
to carry any other path attributes.
6. NLRI encoding
The Network Layer 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 address
prefix. A length of zero indicates a prefix that matches all
(as specified by the address family) addresses (with prefix,
itself, of zero octets).
b) Prefix:
The Prefix field contains address prefixes 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.
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7. Subsequent Address Family Identifier
This document defines the following values for the Subsequent Address
Family Identifier field carried in the MP_REACH_NLRI and
MP_UNREACH_NLRI attributes:
1 - Network Layer Reachability Information used for unicast
forwarding
2 - Network Layer Reachability Information used for multicast
forwarding
3 - Network Layer Reachability Information used for both unicast
and multicast forwarding
This document reserves values 128-255 for vendor-specific
applications.
This document reserves value 0.
Subsequent Address Family Identifiers (other than those reserved for
vendor specific use) are assigned only by the IETF consensus process
and IESG approval.
8. Use of BGP Capability Negotiation
A BGP speaker that uses Multiprotocol Extensions should use the
Capability Negotiation procedures [BGP-CAP] to determine whether the
speaker could use Multiprotocol Extensions with a particular peer.
The fields in the Capabilities Optional Parameter are set as follows.
The Capability Code field is set to 1 (which indicates Multiprotocol
Extensions capabilities). The Capability Length field is set to 4.
The Capability Value field is defined as:
0 7 15 23 31
+-------+-------+-------+-------+
| AFI | Res. | SAFI |
+-------+-------+-------+-------+
The use and meaning of this field is as follow:
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AFI - Address Family Identifier (16 bit), encoded the same way
as in the Multiprotocol Extensions
Res. - Reserved (8 bit) field. Should be set to 0 by the sender
and ignored by the receiver.
SAFI - Subsequent Address Family Identifier (8 bit), encoded
the same way as in the Multiprotocol Extensions.
A speaker that supports multiple <AFI, Sub-AFI> tuples includes them
as multiple Capabilities in the Capabilities Optional Parameter.
To have a bi-directional exchange of routing information for a
particular <AFI, Sub-AFI> between a pair of BGP speakers, each such
speaker must advertise to the other (via the Capability Negotiation
mechanism) the capability to support that particular <AFI, Sub-AFI>
routes.
9. Security Considerations
This extension to BGP does not change the underlying security issues.
10. Acknowledgements
The authors would like to thank members of the IDR Working Group for
their review and comments.
11. References
[BGP-CAP] "Capabilities Negotiation with BGP-4", R. Chandra, J.
Scudder, draft-ietf-idr-bgp4-cap-neg-01.txt, April 1988
[BGP-4] "A Border Gateway Protocol 4 (BGP-4)", Y. Rekhter & T. Li,
RFC1771, March 1995
[IPv4] "Internet Protocol", J. Postel, September 1981
[RFC1700] "Assigned Numbers", J. Reynolds, J. Postel, RFC1700,
October 1994 (see also http://www.iana.org/iana/assignments.html)
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12. Author Information
Tony Bates
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
email: tbates@cisco.com
Ravi Chandra
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
email: rchandra@cisco.com
Dave Katz
Juniper Networks, Inc.
3260 Jay St.
Santa Clara, CA 95054
email: dkatz@jnx.com
Yakov Rekhter
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
email: yakov@cisco.com
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