Internet Engineering Task Force (IETF) B. Decraene
Request for Comments: 6198 France Telecom
Category: Informational P. Francois
ISSN: 2070-1721 UCL
C. Pelsser
IIJ
Z. Ahmad
Orange Business Services
A.J. Elizondo Armengol
Telefonica I+D
T. Takeda
NTT
April 2011
Requirements for the Graceful Shutdown of BGP Sessions
Abstract
The Border Gateway Protocol (BGP) is heavily used in Service Provider
networks for both Internet and BGP/MPLS VPN services. For resiliency
purposes, redundant routers and BGP sessions can be deployed to
reduce the consequences of an Autonomous System Border Router (ASBR)
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 no longer satisfactory for new applications
(e.g., voice over IP, online 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.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6198.
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Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................2
2. Conventions Used in This Document ...............................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 ........................................10
7. References .....................................................10
7.1. Normative References ......................................10
7.2. Informative References ....................................10
Acknowledgments ...................................................11
Appendix A. Reference BGP Topologies ..............................12
A.1. EBGP Topologies ...........................................12
A.2. IBGP Topologies ...........................................15
A.3. Routing Decisions .........................................19
1. Introduction
The Border Gateway Protocol (BGP) [RFC4271] is heavily used in
Service Provider networks for both Internet and BGP/MPLS VPN services
[RFC4364]. For resiliency purposes, redundant routers and BGP
sessions can be deployed to reduce the consequences of an Autonomous
System Border Router (ASBR) 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 Autonomous System (AS), the requirement is to avoid or
reduce customer or peer traffic loss during the BGP convergence.
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Indeed, as an alternate path is available in the 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
withdrawn, and the forwarding 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 there were a sudden link or router
failure and withdraws the prefixes learned over that session, which
may trigger traffic loss. While still being advertised as reachable,
there is no mechanism to advertise to its BGP peers that the prefix
will soon be unreachable. 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 (RRs) as per [RFC4456] to solve
scalability issues bound to Internal BGP (IBGP) full-meshes has
worsened the duration of routing convergence as some route reflectors
may hide the backup path. Thus, depending on RR topology, more IBGP
hops may be involved in the IBGP convergence.
Note that these planned maintenance operations cannot be addressed by
Graceful Restart (GR) 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.
Also, note that some protocols are already considering such a
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.
2. 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].
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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 alternate 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
Service Providers' routers, "ASBR1" and "ASBR2".
ASBR1 and ASBR2 are in the same AS and are owned by the same Service
Provider. Both are IBGP clients of the route reflector R1.
'
AS1 ' AS2
'
/-----------ASBR1---
/ \
/ \
CUST R1
\ /
Z/z \ /
\-----------ASBR2---
'
AS1 ' AS2
'
Figure 1. Dual-Attached Customer
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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 shut down the ASBR1-CUST
link for maintenance purposes. Currently, when the shutdown is
performed on ASBR1, the following steps are performed:
1. ASBR1 withdraws 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 at step 1 when ASBR1 looses its route until step 3
when it discovers a new path.
Note that this is a simplified description for illustrative purposes.
In a bigger AS, multiple steps of BGP convergence may be required to
find and select the best alternate path (e.g., ASBR1 may be chosen
based on a higher LOCAL_PREF, hierarchical route reflectors may be
used, etc.). When multiple BGP routers are involved and plenty of
prefixes are affected, the recovery process can take longer than
application requirements.
3.2. Causes of Packet Loss
The loss of packets during maintenance has two main causes:
- lack of an alternate path on some routers, and
- transient routing inconsistency.
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 regarding the alternate path is the first
target of this requirements document.
Transient routing inconsistencies happen during IBGP convergence
because routers do not simultaneously update their Routing
Information Bases (RIBs) and hence do not simultaneously update their
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Forwarding Information Bases (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), and 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 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 learned 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 learned
over that session) is going to be enabled.
g-shut initiator: the router on which the session(s) shutdown(s) is
(are) performed for 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.
neighbor AS(es): the autonomous system(s) of the g-shut neighbor
router(s).
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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
other known graceful shutdowns under way. Therefore, multiple BGP
graceful shutdowns overlapping within a short time frame are
gracefully handled. Indeed, a given graceful shutdown takes into
account all previous ones.
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 a 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, 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.
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The solution mechanism MUST significantly reduce and, ideally,
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 (External BGP (EBGP), IBGP, IBGP route reflector
client, EBGP confederations, EBGP multi hop, MultiProtocol BGP
extension, etc.) and any address family. If a BGP
implementation allows the closing or enabling of a subset of
Address Family Identifiers (AFIs) carried in an MP-BGP session,
this mechanism MAY be applicable to this subset 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 ASBR 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
Provider Edge - Customer Edge (PE-CE) routing can be performed
by protocols other 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 its 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 ones, mainly
because there is at least an order of magnitude more CE and CE
locations than PE and PE locations. As a consequence, when
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splitting the cost of the solution between the g-shut initiator
and the g-shut neighbor, the solution SHOULD favor a low-cost
solution on the neighbor AS side in order to reduce the impact
on the g-shut neighbor. Impact should be understood as a
generic term that includes first hardware, then software, then
configuration upgrade.
f) Redistribution or advertisement of (static) IP routes into BGP
SHOULD also be covered.
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 re-
established. 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, the use of a
message to signal the end of the BGP convergence, or the
monitoring of 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).
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6. Security Considerations
At the requirements stage, this graceful shutdown mechanism is not
expected to 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 vectors, 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 of one AS to selectively
influence routing decision in the peer AS (inbound Traffic
Engineering) outside of the case of the BGP session shutdown.
Otherwise, the peer AS SHOULD have means to detect such behavior.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", 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.
7.2. Informative References
[RFC5817] Ali, Z., Vasseur, JP., Zamfir, A., and J. Newton,
"Graceful Shutdown in MPLS and Generalized MPLS Traffic
Engineering Networks", RFC 5817, April 2010.
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[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
Acknowledgments
The 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 Ronald
Bonica for their useful discussions on this subject, review, and
comments.
This document has been partly sponsored by the European project IST
AGAVE.
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Appendix A. 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.
A.1. EBGP Topologies
This section describes some frequent BGP topologies used between
ASes. In each figure, a line represents a BGP session.
A.1.1. One 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 the AS2 side, two different routers are used to connect to
AS1.
- On the 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 ASes
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.
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Note that in the 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.
A.1.2. Two ASBRs in AS1 Connected to Two 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.
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A.1.3. Two 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
As the requirement expressed in Section 5 is to advertise the
maintenance only within the initiator and neighbor ASes, 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.
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A.2. IBGP Topologies
This section describes some frequent BGP topologies used within an
AS. In each figure, a line represents a BGP session.
A.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 shut down,
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.
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A.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.
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 shut down,
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.
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A.2.3. Hierarchical Route Reflector
In this topology, hierarchical route reflectors are used to limit the
number of IBGP sessions. There could be more than two levels of
route reflectors and the top-level route reflectors are fully meshed.
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 shut down,
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.
A.2.4. Confederations
In this topology, a confederation of ASes is used to limit the number
of IBGP sessions. Moreover, RRs may be present in the member ASes of
the confederation.
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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 bringing
up of EBGP sessions between member ASes in the confederation in
addition to the graceful shutdown and graceful bringing up of EBGP
sessions between a member-AS and an AS outside of the confederation.
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 ASes AS1A, AS1B, and 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 shut
down, all BGP routers of AS1 need to reroute traffic to ASBR1B.1
before the session between ASBR1A.1 and ASBR2.1 is shut down.
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Similarly, when the session between ASBR1A.1 and ASBR2.1 is
gracefully brought up, all affected routers of AS1 preferring
ASBR1A.1 over ASBR1B.1 need to reroute traffic to ASBR1A.1 before the
less preferred path through ASBR1B.1 is possibly withdrawn.
A.3. Routing Decisions
Here we describe some routing engineering choices that are frequently
used in ASes and that should be supported by the solution.
A.3.1. Hot Potato (IGP Cost)
The ingress router selects the nominal egress ASBR (AS exit point)
based on the IGP cost to reach the BGP next-hop.
A.3.2. Cold Potato (BGP LOCAL_PREF)
The ingress router selects the nominal egress ASBR based on the BGP
LOCAL_PREF value set and advertised by the exit point.
A.3.3. Cold Potato (BGP Preference Set on Ingress)
The ingress router selects the nominal egress ASBR based on
preconfigured policy information. (Typically, this is done 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 the ingress and egress ASBR; this can lead to
persistent forwarding loops.
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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
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
EMail: 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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RFC, FYI, BCP