Requirements for the Graceful Shutdown of BGP Sessions
draft-ietf-grow-bgp-graceful-shutdown-requirements-07
The information below is for an old version of the document that is already published as an RFC.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 6198.
|
|
|---|---|---|---|
| Authors | Cristel Pelsser , Antonio Jose Elizond Armengol , Bruno Decraene , Zubair Ahmad , Tomonori Takeda , Pierre Francois | ||
| Last updated | 2015-10-14 (Latest revision 2011-01-31) | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Informational | ||
| Formats | |||
| Reviews | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | (None) | |
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 6198 (Informational) | |
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(None)
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||
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | Ron Bonica | ||
| Send notices to | (None) |
draft-ietf-grow-bgp-graceful-shutdown-requirements-07
GROW Working Group B. Decraene
Internet-Draft France Telecom
Intended status: Informational P. Francois
UCL
C. Pelsser
IIJ
Z. Ahmad
Orange Business Services
A. J. Elizondo Armengol
Telefonica I+D
T. Takeda
NTT
January 28, 2011
Requirements for the graceful shutdown of BGP sessions
draft-ietf-grow-bgp-graceful-shutdown-requirements-07.txt
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Abstract
The Border Gateway Protocol(BGP) is heavily used in Service Provider
networks both for Internet and BGP/MPLS VPN services. For resiliency
purposes, redundant routers and BGP sessions can be deployed to
reduce the consequences of an AS Border Router or BGP session
breakdown on customers' or peers' traffic. However simply taking down
or even bringing up a BGP session for maintenance purposes may still
induce connectivity losses during the BGP convergence. This is not
satisfactory any more for new applications (e.g. voice over IP, on
line gaming, VPN). Therefore, a solution is required for the graceful
shutdown of a (set of) BGP session(s) in order to limit the amount of
traffic loss during a planned shutdown. This document expresses
requirements for such a solution.
Table of Contents
1. Conventions used in this document...........................3
2. Introduction................................................3
3. Problem statement...........................................4
3.1. Example of undesirable BGP routing behavior.................4
3.2. Causes of packet loss.......................................5
4. Terminology.................................................6
5. Goals and requirements......................................7
6. Security Considerations.....................................9
7. IANA Considerations........................................10
8. References.................................................10
8.1. Normative References.......................................10
8.2. Informative References.....................................10
9. Acknowledgments............................................10
10. Appendix: Reference BGP Topologies.........................12
10.1. EBGP topologies............................................12
10.2. IBGP topologies............................................14
10.3. Routing decisions..........................................17
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1. Conventions used in this document
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 RFC 2119 [RFC2119].
2. Introduction
The Border Gateway Protocol(BGP) [RFC4271] is heavily used in Service
Provider networks both for Internet and BGP/MPLS VPN services
[RFC4364]. For resiliency purposes, redundant routers and BGP
sessions can be deployed to reduce the consequences of an AS Border
Router or BGP session breakdown on customers' or peers' traffic.
We place ourselves in the context where a Service Provider performs a
maintenance operation and needs to shut down one or multiple BGP
peering link(s) or a whole ASBR. If an alternate path is available
within the AS, the requirement is to avoid or reduce customer or peer
traffic loss during the BGP convergence. Indeed, as an alternate path
is available in the Autonomous System (AS), it should be made
possible to reroute the customer or peer traffic on this backup path
before the BGP session(s) is/are torn down, the nominal path is
withdrawn and the forwarding is stopped.
The requirements also cover the subsequent re-establishment of the
BGP session as even this "UP" case can currently trigger route loss
and thus traffic loss at some routers.
BGP [RFC4271] and MP-BGP [RFC4760] do not currently have a mechanism
to gracefully migrate traffic from one BGP next hop to another
without interrupting the flow of traffic. When a BGP session is taken
down, BGP behaves as if it was a sudden link or router failure and
withdraws the prefixes learnt over that session, which may trigger
traffic loss. There is no mechanism to advertise to its BGP peers
that the prefix will soon be unreachable, while still being
reachable. When applicable, such mechanism would reduce or prevent
traffic loss. It would typically be applicable in case of a
maintenance operation requiring the shutdown of a forwarding
resource. Typical examples would be a link or line card maintenance,
replacement or upgrade. It may also be applicable for a software
upgrade as it may involve a firmware reset on the line cards and
hence forwarding interruption.
The introduction of Route Reflectors as per [RFC4456] to solve
scalability issues bound to IBGP full-meshes has worsened the
duration of routing convergence as some route reflectors may hide the
back up path. Thus depending on RR topology more IBGP hops may be
involved in the IBGP convergence.
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Note that these planned maintenance operations cannot be addressed by
Graceful Restart extensions [RFC4724] as GR only applies when the
forwarding is preserved during the control plane restart. On the
contrary, Graceful Shutdown applies when the forwarding is
interrupted.
Note also that some protocols are already considering such graceful
shutdown procedure (e.g. GMPLS in [RFC5817]).
A metric of success is the degree to which such a mechanism
eliminates traffic loss during maintenance operations.
3. Problem statement
As per [RFC4271], when one (or many) BGP session(s) are shut down, a
BGP NOTIFICATION message is sent to the peer and the session is then
closed. A protocol convergence is then triggered both by the local
router and by the peer. Alternate paths to the destination are
selected, if known. If those alternates paths are not known prior to
the BGP session shutdown, additional BGP convergence steps are
required in each AS to search for an alternate path.
This behavior is not satisfactory in a maintenance situation because
the traffic that was directed towards the removed next-hops may be
lost until the end of the BGP convergence. As it is a planned
operation, a make before break solution should be made possible.
As maintenance operations are frequent in large networks [Reliable],
the global availability of the network is significantly impaired by
this BGP maintenance issue.
3.1. Example of undesirable BGP routing behavior
To illustrate these problems, let us consider the following simple
example where one customer router "CUST" is dual-attached to two SP
routers "ASBR1" and "ASBR2".
ASBR1 and ASBR2 are in the same AS and owned by the same service
provider. Both are IBGP client of the route reflector R1.
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'
AS1 ' AS2
'
/-----------ASBR1---
/ \
/ \
CUST R1
\ /
Z/z \ /
\-----------ASBR2---
'
AS1 ' AS2
'
Figure 1. Dual attached customer
Before the maintenance, packets for destination Z/z use the ASBR1-
CUST link because R1 selects ASBR1's route based on the IGP cost.
Let's assume the service provider wants to shutdown the ASBR1-CUST
link for maintenance purposes. Currently, when the shutdown is
performed on ASBR1, the following steps are performed:
1. ASBR1 withdraw its prefix Z/z to its route reflector R1.
2. R1 runs its decision process, selects the route from ASBR2 and
advertises the new path to ASBR1.
3. ASBR1 runs its decision process and recovers the reachability of
Z/z.
Traffic is lost between step 1 when ASBR1 looses its route and step 3
when it discovers a new path.
Note that this is a simplified description for illustrative purpose.
In a bigger AS, multiple steps of BGP convergence may be required to
find and select the best alternate path (e.g. ASBR1 is chosen based
on a higher local pref, hierarchical route reflectors are used...).
When multiple BGP routers are involved and plenty of prefixes are
affected, the recovery process can take longer than applications
requirements.
3.2. Causes of packet loss
The loss of packets during the maintenance has two main causes:
- lack of an alternate path on some routers,
- transient routing inconsistency.
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Some routers may lack an alternate path because another router is
hiding the backup path. This router can be:
- a route reflector only propagating its best path;
- the backup ASBR not advertising the backup path because it prefers
the nominal path.
This lack of knowledge of the alternate path is the first target of
this requirement draft.
Transient routing inconsistencies happen during IBGP convergence
because routers do not simultaneously update their RIBs and hence do
not simultaneously update their FIBs entries. This can lead to
forwarding loops which result in both link congestion and packet
drops. The duration of these transient micro-loops is dependent on
the IBGP topology (e.g. number of Route Reflectors between ingress
and egress ASBR), implementation differences among router platforms
which result in differences in the time taken to update specific
prefix in the FIB, forwarding mode (hop by hop IP forwarding versus
tunneling).
Note that when an IP lookup is only performed on entry to the AS, for
example prior to entry into a tunnel across the AS, micro-loops will
not occur. An example of this is when BGP is being used to as the
routing protocol for MPLS VPN as defined in [RFC4364].
Note that [RFC5715] defines a framework for loop-free convergence. It
has been written in the context of IP Fast ReRoute for link state IGP
[RFC5714] but some concepts are also of interest for BGP convergence.
4. Terminology
g-shut: Graceful SHUTdown. A method for explicitly notifying the BGP
routers that a BGP session (and hence the prefixes learnt over that
session) is going to be disabled.
g-noshut: Graceful NO SHUTdown. A method for explicitly notifying
the BGP routers that a BGP session (and hence the prefixes learnt
over that session) is going to be enabled.
g-shut initiator: the router on which the session(s) shutdown is
(are) performed for the maintenance.
g-shut neighbor: a router that peers with the g-shut initiator
via (one of) the session(s) undergoing maintenance.
Affected prefixes: a prefix initially reached via the peering
link(s) undergoing maintenance.
Affected router: a router reaching an affected prefix via a
peering link undergoing maintenance.
Initiator AS: the autonomous system of the g-shut initiator
router.
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Neighbor AS(es): the autonomous system(s) of the g-shut neighbor
router(s).
5. Goals and requirements
Currently, when a BGP session of the router under maintenance is shut
down, the router removes the routes and then triggers the BGP
convergence on its BGP peers by withdrawing its route.
The goal of BGP graceful shutdown of a (set of) BGP session(s) is to
minimize traffic loss during a planned shutdown. Ideally a solution
should reduce this traffic loss to zero.
Another goal is to minimize and preferably to eliminate packet loss
when the BGP session is re-established following the maintenance.
As the event is known in advance, a make before break solution can be
used in order to initiate the BGP convergence, find and install the
alternate paths before the nominal paths are removed. As a result,
before the nominal BGP session is shut down, all affected routers
learn and use the alternate paths. Those alternate paths are computed
by BGP taking into account the known status of the network which
includes known failures that the network is processing concurrently
with the BGP session graceful shutdown and possibly known other
graceful shutdown under way. Therefore multiple BGP graceful
shutdowns overlapping within a short timeframe are gracefully
handled. Indeed a given graceful shutdown takes into account all
previous ones and previous graceful shutdown are given some time to
adapt to this new one. Then the nominal BGP session can be shut down.
As a result, provided an alternate path with enough remaining
capacity is available, the packets are rerouted before the BGP
session termination and fewer packets (possibly none) are lost during
the BGP convergence process since at any time, all routers have a
valid path.
From the above goals we can derive the following requirements:
a) A mechanism to advertise the maintenance action to all affected
routers is REQUIRED. Such mechanism may be either implicit or
explicit. Note that affected routers can be located both in the local
AS and in neighboring ASes. Note also that the maintenance action can
either be the shutdown of a BGP session or the establishment of a BGP
session.
The mechanism SHOULD allow BGP routers to minimize and preferably to
eliminate packet loss when a path is removed or advertised. In
particular, it SHOULD be ensured that the old path is not removed
from the routing tables of the affected routers before the new path
is known.
The solution mechanism MUST significantly reduce and ideally
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eliminate packet loss. A trade off may be made between the degree of
packet loss and the simplicity of the solution.
b) An Internet wide convergence is OPTIONAL. However if the
initiator AS and the neighbor AS(es) have a backup path, they SHOULD
be able to gracefully converge before the nominal path is shut down.
c) The proposed solution SHOULD be applicable to any kind of BGP
sessions (EBGP, IBGP, IBGP route reflector client, EBGP
confederations, EBGP multi hop, MultiProtocol BGP extension...) and
any address family. If a BGP implementation allows closing or
enabling a sub-set of AFIs carried in a MP-BGP session, this
mechanism MAY be applicable to this sub-set of AFIs.
Depending on the kind of session, there may be some variations in the
proposed solution in order to fulfill the requirements.
The following cases should be handled in priority:
- The shutdown of an inter-AS link and therefore the shutdown of an
eBGP session;
- The shutdown of an AS Border Router and therefore the shutdown of
all its BGP sessions.
Service Providers and platforms implementing a graceful shutdown
solution should note that in BGP/MPLS VPN as per [RFC4364], the PE-CE
routing can be performed by other protocols than BGP (e.g. static
routes, RIPv2, OSPF, IS-IS). This is out of scope of this document.
d) The proposed solution SHOULD NOT change the BGP convergence
behavior for the ASes exterior to the maintenance process, namely
ASes other than the initiator AS and it(s) neighbor AS(es).
e) An incremental deployment on a per AS or per BGP session basis
MUST be made possible. In case of partial deployment the proposed
solution SHOULD incrementally improve the maintenance process.
It should be noted that in an inter domain relation, one AS may have
more incentive to use graceful shutdown than the other. Similarly, in
a BGP/MPLS VPN environment, it's much easier to upgrade the PE
routers than the CE mainly because there is at least an order of
magnitude more CE and CE locations than PE and PE locations. As a
consequence, when splitting the cost of the solution between the g-
shut initiator and the g-shut neighbour the solution SHOULD favour a
low cost solution on the neighbour AS side in order to reduce the
impact on the g-shut neighbour. Impact should be understood as a
generic term which includes first hardware, then software, then
configuration upgrade.
f) Redistribution or advertisement of (static) IP routes into BGP
SHOULD also be covered.
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g) The proposed solution MAY be designed in order to avoid
transient forwarding loops. Indeed, forwarding loops increase packet
transit delay and may lead to link saturation.
h) The specific procedure SHOULD end when the BGP session is closed
following the g-shut and once the BGP session is gracefully opened
following the g-noshut. In the end, once the planned maintenance is
finished the nominal BGP routing MUST be reestablished.
The duration of the g-shut procedure, and hence the time before the
BGP session is safely closed SHOULD be discussed by the solution
document. Examples of possible solutions are the use of a pre-
configured timer, of a message to signal the end of the BGP
convergence or monitoring the traffic on the g-shut interface.
i) The solution SHOULD be simple and simple to operate. Hence it
MAY only cover a subset of the cases. As a consequence, most of the
above requirements are expressed as "SHOULD" rather than "MUST".
The metrics to evaluate and compare the proposed solutions are:
- The duration of the remaining loss of connectivity when the BGP
session is brought down or up;
- The applicability to a wide range of BGP and network topologies;
- The simplicity;
- The duration of transient forwarding loops;
- The additional load introduced in BGP (e.g. BGP messages sent to
peer routers, peer ASes, the Internet).
6. Security Considerations
At the requirements stage, this graceful shutdown mechanism is
expected to not affect the security of the BGP protocol, especially
if it can be kept simple. No new sessions are required and the
additional ability to signal the graceful shutdown is not expected to
bring additional attack vector as BGP neighbors already have the
ability to send incorrect or misleading information or even shut down
the session.
Security considerations MUST be addressed by the proposed
solutions. In particular they SHOULD address the issues of bogus
g-shut messages and how they would affect the network(s), as well
as the impact of hiding a g-shut message so that g-shut is not
performed.
The solution SHOULD NOT increase the ability for one AS to
selectively influence routing decision in the peer AS (inbound
Traffic Engineering) outside the case of the BGP session
shutdown. Otherwise, the peer AS SHOULD have means to detect such
behavior.
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7. IANA Considerations
This document has no actions for IANA.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4271] Rekhter, Y. and T. Li, "A Border Gateway protocol 4
(BGP)", RFC 4271, January 2006.
[RFC4760] Bates, T., Chandra, R., Katz, D. and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760 January
2007.
[RFC4456] Bates, T., Chen E. and R. Chandra "BGP Route Reflection:
An Alternative to Full Mesh Internal BGP (IBGP)", RFC
4456 April 2006.
[RFC4364] Rosen, E. and Y. Rekhter "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364 February 2006.
8.2. Informative References
[RFC5817] Ali, Z., Vasseur, J.P., Zamfir, A. and J. Newton,
"Graceful Shutdown in MPLS and Generalized MPLS Traffic
Engineering Networks", RFC 5817, April 2010.
[RFC5715] Shand, M. and S. Bryant, "A Framework for Loop-Free
Convergence", RFC 5715, January 2010.
[RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework", RFC
5714, January 2010.
[RFC4724] Sangli, S., Chen, E., Fernando, R., Scudder, J. and Y.
Rekhter, "Graceful Restart Mechanism for BGP", RFC
4724, January 2007.
[Reliable] Network Strategy Partners, LLC. "Reliable IP Nodes: A
prerequisite to profitable IP services", November 2002.
http://www.nspllc.com/NewPages/Reliable_IP_Nodes.pdf
9. Acknowledgments
Authors would like to thank Nicolas Dubois, Benoit Fondeviole,
Christian Jacquenet, Olivier Bonaventure, Steve Uhlig, Xavier
Vinet, Vincent Gillet, Jean-Louis le Roux, Pierre Alain Coste and
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Ronald Bonica for the useful discussions on this subject, their
review and comments.
This draft has been partly sponsored by the European project IST
AGAVE.
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10. Appendix: Reference BGP Topologies
This section describes some frequent BGP topologies used both within
the AS (IBGP) and between ASes (EBGP). Solutions should be applicable
to the following topologies and their combinations.
10.1. EBGP topologies
This section describes some frequent BGP topologies used between
ASes. In each figure, a line represents a BGP session.
10.1.1. 1 ASBR in AS1 connected to two ASBRs in the neighboring AS2
In this topology we have an asymmetric protection scheme between
AS1 and AS2:
- On AS2 side, two different routers are used to connect to AS1.
- On AS1 side, one single router with two BGP sessions is used.
'
AS1 ' AS2
'
/----------- ASBR2.1
/ '
/ '
ASBR1.1 '
\ '
\ '
\----------- ASBR2.2
'
'
AS1 ' AS2
'
Figure 2. EBGP topology with redundant ASBR in one of the AS.
BGP graceful shutdown is expected to be applicable for the
maintenance of:
- one of the routers of AS2;
- one link between AS1 and AS2, performed either on an AS1 or AS2
router.
Note that in case of maintenance of the whole router, all its BGP
sessions need to be gracefully shutdown at the beginning of the
maintenance and gracefully brought up at the end of the
maintenance.
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10.1.2. 2 ASBRs in AS1 connected to 2 ASBRs in AS2
In this topology we have a symmetric protection scheme between
AS1 and AS2: on both sides, two different routers are used to
connect AS1 to AS2.
'
AS1 ' AS2
'
ASBR1.1----------- ASBR2.1
'
'
'
'
'
ASBR1.2----------- ASBR2.2
'
AS1 ' AS2
'
Figure 3. EBGP topology with redundant ASBRs in both ASes
BGP graceful shutdown is expected to be applicable for the
maintenance of:
- any of the ASBR routers (in AS1 or AS2);
- one link between AS1 and AS2 performed either on an AS1 or AS2
router.
10.1.3. 2 ASBRs in AS2 each connected to two different ASes
In this topology at least three ASes are involved.
'
AS1 ' AS2
'
ASBR1.1----------- ASBR2.1
| '
| '
'''''|''''''''''
| '
| '
ASBR3.1----------- ASBR2.2
'
AS3 ' AS2
Figure 4. EBGP topology of a dual homed customer
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As the requirements expressed in section 5 is to advertise the
maintenance only within the initiator and neighbor ASes, but not
Internet wide, BGP graceful shutdown solutions may not be
applicable to this topology. Depending on which routes are
exchanged between these ASes, some protection for some of the
traffic may be possible.
For instance if ASBR2.2 performs a maintenance affecting ASBR3.1 then
ASBR3.1 will be notified. However ASBR1.1 may not be notified of the
maintenance of the eBGP session between ASBR3.1 and ASBR2.2.
10.2. IBGP topologies
This section describes some frequent BGP topologies used within an
AS. In each figure, a line represents a BGP session.
10.2.1. IBGP Full-Mesh
In this topology we have a full mesh of IBGP sessions:
P1 ----- P2
| \ / |
| \ / |
| \/ | AS1
| /\ |
| / \ |
| / \ |
ASBR1.1--ASBR1.2
\ /
\ /
''''''\'''/''''''''''''
\ / AS2
ASBR2.1
Figure 5. IBGP full mesh
When the session between ASBR1.1 and ASBR2.1 is gracefully
shutdown, it is required that all affected routers of AS1 reroute
traffic to ASBR1.2 before the session between ASBR1.1 and ASBR2.1
is shut down.
Similarly, when the session between ASBR1.1 and ASBR2.1 is
gracefully brought up, all affected routers of AS1 preferring
ASBR1.1 over ASBR1.2 need to reroute traffic to ASBR1.1 before the
less preferred path through ASBR1.2 is possibly withdrawn.
10.2.2. Route Reflector
In this topology, route reflectors are used to limit the number of
IBGP sessions. There is a single level of route reflectors and the
route reflectors are fully meshed.
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P1 (RR)-- P2 (RR)
| \ / |
| \ / |
| \ / | AS1
| \/ |
| /\ |
| / \ |
| / \ |
| / \ |
ASBR1.1 ASBR1.2
\ /
\ /
''''''\''''''/''''''''''''
\ /
\ / AS2
ASBR2.1
Figure 6. Route Reflector
When the session between ASBR1.1 and ASBR2.1 is gracefully
shutdown, all BGP routers of AS1 need to reroute traffic to
ASBR1.2 before the session between ASBR1.1 and ASBR2.1 is shut
down.
Similarly, when the session between ASBR1.1 and ASBR2.1 is
gracefully brought up, all affected routers of AS1 preferring
ASBR1.1 over ASBR1.2 need to reroute traffic to ASBR1.1 before the
less preferred path through ASBR1.2 is possibly withdrawn.
10.2.3. hierarchical Route Reflector
In this topology, hierarchical route reflectors are used to limit
the number of IBGP sessions. There could me more than two levels
of route reflectors and the top level route reflectors are fully
meshed.
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P1 (RR) -------- P2 (RR)
| |
| |
| | AS1
| |
| |
P3 (RR) P4 (RR)
| |
| |
| | AS1
| |
| |
ASBR1.1 ASBR1.2
\ /
\ /
''''''\'''''''''/''''''''''''
\ /
\ / AS2
ASBR2.1
Figure 7. Hierarchical Route Reflector
When the session between ASBR1.1 and ASBR2.1 is gracefully
shutdown, all BGP routers of AS1 need to reroute traffic to
ASBR1.2 before the session between ASBR1.1 and ASBR2.1 is shut
down.
Similarly, when the session between ASBR1.1 and ASBR2.1 is
gracefully brought up, all affected routers of AS1 preferring
ASBR1.1 over ASBR1.2 need to reroute traffic to ASBR1.1 before the
less preferred path through ASBR1.2 is possibly withdrawn.
10.2.4. Confederations
In this topology, a confederation of ASs is used to limit the number
of IBGP sessions. Moreover, RRs may be present in the member ASs of
the confederation.
Confederations may be run with different sub-options. Regarding the
IGP, each member AS can run its own IGP or they can all share the
same IGP. Regarding BGP, local_pref may or may not cross the member
AS boundaries.
A solution should support the graceful shutdown and graceful bring up
of EBGP sessions between member-ASs in the confederation in addition
to the graceful shutdown and graceful bring up of EBGP sessions
between a member-AS and an AS outside of the confederation.
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ASBR1C.1 ---------- ASBR1C.2
| |
| |
| AS1C |
| |
| |
"""|"""""""""""""""""""|"""
| " |
ASBR1A.2 " ASBR1B.2
| " |
| " |
| AS1A " AS1B | AS1
| " |
| " |
ASBR1A.1 " ASBR1B.1
\ " /
\ " /
''''''\'''''''''''''/''''''''''''
\ /
\ / AS2
ASBR2.1
Figure 8. Confederation
In the above figure, member-AS AS1A, AS1B, AS1C belong to a
confederation of ASes in AS1. AS1A and AS1B are connected to AS2.
In normal operation, for the traffic toward AS2,
. AS1A sends the traffic directly to AS2 through ASBR1A.1
. AS1B sends the traffic directly to AS2 through ASBR1B.1
. AS1C load balances the traffic between AS1A and AS1B
When the session between ASBR1A.1 and ASBR2.1 is gracefully shutdown,
all BGP routers of AS1 need to reroute traffic to ASBR1B.1 before the
session between ASBR1A.1 and ASBR2.1 is shut down.
Similarly, when the session between ASBR1A.1 and ASBR2.1 is
gracefully brought up, all affected routers of AS1 preferring
ASBR1A.1 over ASBR1.2 need to reroute traffic to ASBR1A.1 before the
less preferred path through ASBR1.2 is possibly withdrawn.
10.3. Routing decisions
We describe here some routing engineering choices that are
frequently used in ASes and that should be supported by the
solution.
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10.3.1. Hot potato (IGP cost)
Ingress router selects the nominal egress ASBR (AS exit point)
based on the IGP cost to reach the BGP next-hop.
10.3.2. Cold potato (BGP local preference)
Ingress router selects the nominal egress ASBR based on the BGP
local LOCAL_PREF value set and advertised by the exit point.
10.3.3. Cold potato (BGP preference set on ingress)
Ingress router selects the nominal egress ASBR based on
preconfigured policy information. (Typically by locally setting
the BGP local pref based on the BGP communities attached on the
routes).
As per [RFC4271], note that if tunnels are not used to forward
packets between ingress and egress ASBR, this can lead to
persistent forwarding loops.
Authors' Addresses
Bruno Decraene
France Telecom
38-40 rue du General Leclerc
92794 Issy Moulineaux cedex 9
France
Email: bruno.decraene@orange-ftgroup.com
Pierre Francois
Universite catholique de Louvain
Place Ste Barbe, 2
Louvain-la-Neuve 1348
BE
Email: francois@info.ucl.ac.be
Cristel Pelsser
Internet Initiative Japan
Jinbocho Mitsui Building
1-105 Kanda jinbo-cho
Chiyoda-ku, Tokyo 101-0051
Japan
Email: cristel@iij.ad.jp
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Zubair Ahmad
Orange Business Services
13775 McLearen Road, Oak Hill VA 20171
USA
Email: zubair.ahmad@orange-ftgroup.com
Antonio Jose Elizondo Armengol
Division de Analisis Tecnologicos
Technology Analysis Division
Telefonica I+D
C/ Emilio Vargas 6
28043, Madrid
E-mail: ajea@tid.es
Tomonori Takeda
NTT Corporation
9-11, Midori-Cho 3 Chrome
Musashino-Shi, Tokyo 180-8585
Japan
Email: takeda.tomonori@lab.ntt.co.jp
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