draft-ietf-grow-bgp-gshut-00.txt   draft-ietf-grow-bgp-gshut-01.txt 
Network Working Group Pierre Francois Network Working Group Pierre Francois
Internet-Draft Universite catholique de Louvain Internet-Draft Universite catholique de Louvain
Intended status: Informational Bruno Decraene Intended status: Informational Bruno Decraene
Expires: December 17, 2009 France Telecom Expires: April 29, 2010 France Telecom
Cristel Pelsser Cristel Pelsser
NTT Corporation Internet Initiative Japan
Clarence Filsfils Clarence Filsfils
Cisco Systems Cisco Systems
June 15, 2009 October 26, 2009
Graceful BGP session shutdown Graceful BGP session shutdown
draft-ietf-grow-bgp-gshut-00 draft-ietf-grow-bgp-gshut-01
Status of this Memo Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the This Internet-Draft is submitted to IETF in full conformance with the
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Trust the right to allow modifications of such material outside the Trust the right to allow modifications of such material outside the
IETF Standards Process. Without obtaining an adequate license from IETF Standards Process. Without obtaining an adequate license from
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and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
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This Internet-Draft will expire on December 17, 2009. This Internet-Draft will expire on April 29, 2010.
Copyright Notice Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
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Please review these documents carefully, as they describe your rights Please review these documents carefully, as they describe your rights
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1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Packet loss upon manual eBGP session shutdown . . . . . . . . 5 3. Packet loss upon manual eBGP session shutdown . . . . . . . . 5
4. Practices to avoid packet losses . . . . . . . . . . . . . . . 5 4. Practices to avoid packet losses . . . . . . . . . . . . . . . 5
4.1. Improving availability of alternate paths . . . . . . . . 6 4.1. Improving availability of alternate paths . . . . . . . . 6
4.2. Graceful shutdown procedures for eBGP sessions . . . . . . 6 4.2. Graceful shutdown procedures for eBGP sessions . . . . . . 6
4.2.1. Outbound traffic . . . . . . . . . . . . . . . . . . . 6 4.2.1. Outbound traffic . . . . . . . . . . . . . . . . . . . 6
4.2.2. Inbound traffic . . . . . . . . . . . . . . . . . . . 7 4.2.2. Inbound traffic . . . . . . . . . . . . . . . . . . . 7
4.3. Graceful shutdown procedures for iBGP sessions . . . . . . 9 4.3. Graceful shutdown procedures for iBGP sessions . . . . . . 9
5. Forwarding modes and forwarding loops . . . . . . . . . . . . 9 5. Forwarding modes and forwarding loops . . . . . . . . . . . . 10
6. Dealing with Internet policies . . . . . . . . . . . . . . . . 10 6. Dealing with Internet policies . . . . . . . . . . . . . . . . 10
7. Effect of the g-shut procedure on the convergence . . . . . . 10 7. Link Up cases . . . . . . . . . . . . . . . . . . . . . . . . 11
7.1. Maintenance of an eBGP session . . . . . . . . . . . . . . 10 7.1. Unreachability local to the ASBR . . . . . . . . . . . . . 11
7.1.1. Propagation on the other eBGP sessions of the 7.2. iBGP convergence . . . . . . . . . . . . . . . . . . . . . 11
g-shut initiator . . . . . . . . . . . . . . . . . . . 10 8. IANA considerations . . . . . . . . . . . . . . . . . . . . . 12
7.1.2. Propagation on the other iBGP sessions of the 9. Security Considerations . . . . . . . . . . . . . . . . . . . 12
g-shut initiator . . . . . . . . . . . . . . . . . . . 11 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 13
7.1.3. Propagation of updates in an iBGP full-mesh . . . . . 11 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
7.1.4. Propagation of updates from iBGP to iBGP in a RR Appendix A. Summary of operations . . . . . . . . . . . . . . . . 13
hierarchy . . . . . . . . . . . . . . . . . . . . . . 11 A.1. Pre-configuration . . . . . . . . . . . . . . . . . . . . 13
7.2. Maintenance of an iBGP session . . . . . . . . . . . . . . 12 A.2. Operations at maintenance time . . . . . . . . . . . . . . 14
7.3. Applicability of the g-shut procedure . . . . . . . . . . 13 Appendix B. Alternative techniques with limited applicability . . 14
7.4. Summary of operations . . . . . . . . . . . . . . . . . . 13 B.1. In-filter reconfiguration . . . . . . . . . . . . . . . . 14
7.4.1. Pre-configuration . . . . . . . . . . . . . . . . . . 13 B.2. Multi Exit Discriminator tweaking . . . . . . . . . . . . 15
7.4.2. Operations at maintenance time . . . . . . . . . . . . 13 B.3. IGP distance Poisoning . . . . . . . . . . . . . . . . . . 16
8. Link Up cases . . . . . . . . . . . . . . . . . . . . . . . . 13 Appendix C. Effect of the g-shut procedure on the convergence . . 16
8.1. Unreachability local to the ASBR . . . . . . . . . . . . . 13 C.1. Maintenance of an eBGP session . . . . . . . . . . . . . . 16
8.2. iBGP convergence . . . . . . . . . . . . . . . . . . . . . 14 C.1.1. Propagation on the other eBGP sessions of the
9. Alternative techniques with limited applicability . . . . . . 15 g-shut initiator . . . . . . . . . . . . . . . . . . . 16
9.1. In-filter reconfiguration . . . . . . . . . . . . . . . . 15 C.1.2. Propagation on the other iBGP sessions of the
9.2. Multi Exit Discriminator tweaking . . . . . . . . . . . . 16 g-shut initiator . . . . . . . . . . . . . . . . . . . 16
9.3. IGP distance Poisoning . . . . . . . . . . . . . . . . . . 16 C.1.3. Propagation of updates in an iBGP full-mesh . . . . . 17
10. IANA considerations . . . . . . . . . . . . . . . . . . . . . 16 C.1.4. Propagation of updates from iBGP to iBGP in a RR
11. Security Considerations . . . . . . . . . . . . . . . . . . . 17 hierarchy . . . . . . . . . . . . . . . . . . . . . . 17
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17 C.2. Maintenance of an iBGP session . . . . . . . . . . . . . . 18
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17
1. Introduction 1. Introduction
Routing changes in BGP can be caused by planned, manual, maintenance Routing changes in BGP can be caused by planned, manual, maintenance
operations. This document discusses operational procedures to be operations. This document discusses operational procedures to be
applied in order to reduce or eliminate losses of packets during the applied in order to reduce or eliminate losses of packets during the
maintenance. These losses come from the transient lack of maintenance. These losses come from the transient lack of
reachability during the BGP convergence following the shutdown of an reachability during the BGP convergence following the shutdown of an
eBGP peering session between two Autonomous System Border Routers eBGP peering session between two Autonomous System Border Routers
(ASBR). (ASBR).
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during the convergence process. during the convergence process.
The goal of the document is to meet the requirements described in The goal of the document is to meet the requirements described in
[REQS] at best, without changing the BGP protocol or BGP [REQS] at best, without changing the BGP protocol or BGP
implementations. implementations.
Still, it explains why reserving a community value for the purpose of Still, it explains why reserving a community value for the purpose of
BGP session graceful shutdown would reduce the management overhead BGP session graceful shutdown would reduce the management overhead
bound with the solution. It would also allow vendors to provide an bound with the solution. It would also allow vendors to provide an
automatic graceful shutdown mechanism that does not require any automatic graceful shutdown mechanism that does not require any
configuration at maintenance time. router reconfiguration at maintenance time.
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. Terminology 2. Terminology
g-shut initiator : a router on which the session shutdown is g-shut initiator : a router on which the session shutdown is
performed for the maintenance. performed for the maintenance.
g-shut neighbor : a router that peers with the g-shut initiator via g-shut neighbor : a router that peers with the g-shut initiator via
(one of) the session(s) to be shut down. (one of) the session(s) to be shut down.
Note that for the link-up case, we will refer to these nodes as g-no- Note that for the link-up case, we will refer to these nodes as g-no-
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Neighbor AS : the Autonomous System of the g-shut neighbor. Neighbor AS : the Autonomous System of the g-shut neighbor.
Affected path / Nominal / pre-convergence path : a BGP path via the Affected path / Nominal / pre-convergence path : a BGP path via the
peering link(s) undergoing the maintenance. This path will no longer peering link(s) undergoing the maintenance. This path will no longer
exist after the shutdown. exist after the shutdown.
Affected prefix : a prefix initially reached via an affected path. Affected prefix : a prefix initially reached via an affected path.
Affected router : a router having an affected prefix. Affected router : a router having an affected prefix.
Backup / alternate / post-convergence path : a path toward an Backup / alternate / post-convergence path : a path towards an
affected prefix that will be selected as the best path by an affected affected prefix that will be selected as the best path by an affected
router, when the link is shut down and the BGP convergence is router, when the link is shut down and the BGP convergence is
completed. completed.
Transient alternate path : a path towards an affected prefix that may Transient alternate path : a path towards an affected prefix that may
be transiently selected as best by an affected router during the be transiently selected as best by an affected router during the
convergence process but that is not a post-convergence path. convergence process but that is not a post-convergence path.
Loss of Connectivity (LoC) : the state when a router has no path Loss of Connectivity (LoC) : the state when a router has no path
towards an affected prefix. towards an affected prefix.
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This section describes means for an ISP to reduce the transient loss This section describes means for an ISP to reduce the transient loss
of packets upon a manual shutdown of a BGP session. of packets upon a manual shutdown of a BGP session.
4.1. Improving availability of alternate paths 4.1. Improving availability of alternate paths
All solutions that increase the availability of alternate BGP paths All solutions that increase the availability of alternate BGP paths
in routers performing packet lookups in BGP tables [BestExternal] in routers performing packet lookups in BGP tables [BestExternal]
[AddPath] help in reducing the LoC bound with manual shutdown of eBGP [AddPath] help in reducing the LoC bound with manual shutdown of eBGP
sessions. sessions.
One solution increasing diversity in such a way that, at any single One of such solutions increasing diversity in such a way that, at any
step of the convergence process following the eBGP session shutdown, single step of the convergence process following the eBGP session
a BGP router does not receive a message withdrawing the only path it shutdown, a BGP router does not receive a message withdrawing the
currently knows for a given NLRI, allows for a simplified g-shut only path it currently knows for a given NLRI, allows for a
procedure. This simplified procedure would only tackle potential LoC simplified g-shut procedure. This simplified procedure would only
for the inbound traffic. tackle potential LoC for the inbound traffic.
Using advertise-best-external [BestExternal] on ASBRs and RRs helps Using advertise-best-external [BestExternal] on ASBRs and RRs helps
in avoiding lack of alternate paths in route reflectors upon a in avoiding lack of alternate paths in route reflectors upon a
convergence. Hence it reduces the LoC duration for the outbound convergence. Hence it reduces the LoC duration for the outbound
traffic of the ISP upon an eBGP Session shutdown by reducing the iBGP traffic of the ISP upon an eBGP Session shutdown by reducing the iBGP
path hunting. path hunting.
Still it does not ensure that BGP routers will always have at least Still it does not ensure that BGP routers will always have at least
one path towards affected prefixes during the convergence following one path towards affected prefixes during the convergence following
the event. This property may be verified in future revisions of the event. This property may be verified in future revisions of
[BestExternal], notably of its Section 4, hence the current proposal [BestExternal], notably of its Section 3, hence the current proposal
will be updated accordingly. will be updated accordingly.
Increasing diversity with [AddPath] might lead to the respect of this Increasing diversity with [AddPath] might lead to the respect of this
property, depending on the path propagation decision process that property, depending on the path propagation decision process that
add-path compliant routers would use. add-path compliant routers would use.
Note that the LoC for the inbound traffic of the maintained router, Note that the LoC for the inbound traffic of the maintained router,
induced by a lack of alternate path propagation within the iBGP induced by a lack of alternate path propagation within the iBGP
topology of a neighboring AS is not under the control of the operator topology of a neighboring AS is not under the control of the operator
performing the maintenance, hence the procedure described in performing the maintenance, hence the procedure described in
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switch to another path, as the input to the BGP decision process of switch to another path, as the input to the BGP decision process of
that router does not change. As a consequence, the g-shut initiator that router does not change. As a consequence, the g-shut initiator
will not send a withdraw message over its iBGP sessions when it will not send a withdraw message over its iBGP sessions when it
receives an alternate path over an iBGP session. It will however receives an alternate path over an iBGP session. It will however
modify the local-pref of the affected paths so that upstream routers modify the local-pref of the affected paths so that upstream routers
will switch to alternate ones. will switch to alternate ones.
When the actual shutdown of the session is performed, the g-shut When the actual shutdown of the session is performed, the g-shut
initiator will itself switch to the alternate paths. initiator will itself switch to the alternate paths.
In cases some BGP speakers in the AS override the local-pref
attribute of paths received over iBGP sessions, the procedure
described above will not work. In such cases, the recommended
procedure is to tag the paths sent over the iBGP sessions of the
g-shut initiator with an AS specific community. This AS specific
community should lead to the setting of a low local-pref value. To
be effective, the configuration related to this community MUST
supplant or be applied after the already configured local-pref
overriding.
4.2.2. Inbound traffic 4.2.2. Inbound traffic
The solution described for the outbound traffic can be applied at the The solution described for the outbound traffic can be applied at the
neighbor AS. This can be done either "manually" or by using a neighbor AS. This can be done either "manually" or by using a
community value dedicated to this task. community value dedicated to this task.
4.2.2.1. Phone call 4.2.2.1. Phone call
The operator performing the maintenance of the eBGP session can The operator performing the maintenance of the eBGP session can
contact the operator at the other side of the peering link, and let contact the operator at the other side of the peering link, and let
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4.2.2.2.2. Operational action upon maintenance 4.2.2.2.2. Operational action upon maintenance
Upon the manual shutdown, the output filter associated with the Upon the manual shutdown, the output filter associated with the
maintained eBGP session will be modified on the g-shut initiator so maintained eBGP session will be modified on the g-shut initiator so
as to tag all the paths advertised over the session with the GSHUT as to tag all the paths advertised over the session with the GSHUT
community. community.
4.2.2.2.3. Transitivity of the community 4.2.2.2.3. Transitivity of the community
If the GSHUT community is an extended community, it SHOULD be set non If the GSHUT community is an extended community, it SHOULD be chosen
transitive. non-transitive. In that case, the clarification described in
[Clarification4360] is required.
If a normal community is used, this community SHOULD be removed from If a regular community is used, this community SHOULD be removed from
the path by the ASBR of the peer receiving it. If not, the GSHUT the path when the path is propagated over eBGP sessions.
community MAY be removed from the path by all the ASBRs of the
neighboring AS, before propagating the path to other peers.
Not propagating the community further in the Internet reduces the Not propagating the community further in the Internet reduces the
amount of BGP churn and avoids rerouting in distant ASes that would amount of BGP churn and avoids rerouting in distant ASes that would
also recognize this community value. In other words, it helps also recognize this community value. In other words, from a routing
concealing the convergence at the maintenance location. stability perspective, it helps concealing the convergence at the
maintenance location. From a security perspective, it prevents
malignant ASes from using the community over paths propagated through
intermediate ASes that do not support the feature, in order to
perform inbound traffic engineering at the first AS recognizing the
community.
ASes which support the g-shut procedure SHOULD remove the community
value(s) that they use for g-shut from the paths received from
neighboring ASes that do not support the procedure or to whom the
service is not provided.
There are cases where an interdomain exploration is to be performed There are cases where an interdomain exploration is to be performed
to recover the reachability, e.g., in the case of a shutdown in to recover the reachability, e.g., in the case of a shutdown in
confederations where the alternate paths will be found in another AS confederations where the alternate paths will be found in another AS
of the confederation. In such scenarios, the community value SHOULD of the confederation. In such scenarios, the community value SHOULD
be allowed to transit through the confederation but MAY be removed be allowed to transit through the confederation but SHOULD be removed
from the paths advertised outside of the confederation. from the paths advertised outside of the confederation.
When the local-pref value of a path is conserved upon its propagation When the local-pref value of a path is conserved upon its propagation
from one AS of the confederation to the other, there is no need to from one AS of the confederation to the other, there is no need to
have the GSHUT community be propagated throughout that confederation. have the GSHUT community be propagated throughout that confederation.
4.2.2.2.4. Easing the configuration for G-SHUT 4.2.2.2.4. Easing the configuration for G-SHUT
From a configuration burden viewpoint, it would be much easier to From a configuration burden viewpoint, it is much easier to use a
reserve a value for the GSHUT community. single dedicated value for the GSHUT community.
First, on the g-shut initiator, an operator would have a single First, on the g-shut initiator, an operator would have a single
configuration rule to be applied at the maintenance time, which would configuration rule to be applied at the maintenance time, which would
not depend on the identity of its peer. This would make the not depend on the identity of its peer. This would make the
maintenance operations less error prone. maintenance operations less error prone.
Second, on the g-shut neighbor, a simple filter related to g-shut can Second, on the g-shut neighbor, a simple filter related to g-shut can
be applied to all iBGP sessions. Additionnaly, this filter doesn't be applied to all iBGP sessions. Additionnaly, this filter does not
need to be updated each time neighboring ASes are added or removed. need to be updated each time neighboring ASes are added or removed.
The FCFS community value 0xFFFF0000 has been reserved for this
purpose [BGPWKC].
4.3. Graceful shutdown procedures for iBGP sessions 4.3. Graceful shutdown procedures for iBGP sessions
If the iBGP topology is viable after the maintenance of the session, If the iBGP topology is viable after the maintenance of the session,
i.e, if all BGP speakers of the AS have an iBGP signaling path for i.e, if all BGP speakers of the AS have an iBGP signaling path for
all prefixes advertised on this g-shut iBGP session, then the all prefixes advertised on this g-shut iBGP session, then the
shutdown of an iBGP session does not lead to transient shutdown of an iBGP session does not lead to transient
unreachability. unreachability.
However, in the case of a shutdown of a router, a reconfiguration of However, in the case of a shutdown of a router, a reconfiguration of
the out-filters of the g-shut initiator should be performed to set a the out-filters of the g-shut initiator MAY be performed to set a low
low local-pref value for the paths originated by the g-shut initiator local-pref value for the paths originated by the g-shut initiator
(e.g, BGP aggregates redistributed from other protocols, including (e.g, BGP aggregates redistributed from other protocols, including
static routes). static routes).
This behavior is equivalent to the recommended behavior for paths This behavior is equivalent to the recommended behavior for paths
"redistributed" from eBGP sessions to iBGP sessions in the case of "redistributed" from eBGP sessions to iBGP sessions in the case of
the shutdown of an ASBR. the shutdown of an ASBR.
5. Forwarding modes and forwarding loops 5. Forwarding modes and forwarding loops
If the AS applying the solution does not rely on encapsulation to If the AS applying the solution does not rely on encapsulation to
forward packets from the Ingress Border Router to the Egress Border forward packets from the Ingress Border Router to the Egress Border
Router, then transient forwarding loops and consequent packet losses Router, then transient forwarding loops and consequent packet losses
can occur during the convergence process, even if the procedure can occur during the convergence process, even if the procedure
described above is applied. Hence if zero LoC is required, described above is applied. Hence if zero LoC is required,
encapsulation is required between ASBRs of the AS. encapsulation is required between ASBRs of the AS.
Using the out-filter reconfiguration avoids the forwarding loops Using the out-filter reconfiguration avoids the forwarding loops
between the g-shut initiator and its directly connected upstream between the g-shut initiator and its directly connected upstream
neighbors. Indeed, when this reconfiguration is applied, the g-shut neighboring routers. Indeed, when this reconfiguration is applied,
initiator keeps using its own external path and lets the upstream the g-shut initiator keeps using its own external path and lets the
routers converge to the alternate ones. During this phase, no upstream routers converge to the alternate ones. During this phase,
forwarding loops can occur between the g-shut initiator and its no forwarding loops can occur between the g-shut initiator and its
upstream neighbors as the g-shut initiator keeps using the affected upstream neighbors as the g-shut initiator keeps using the affected
paths via its eBGP peering links. When all the upstream routers have paths via its eBGP peering links. When all the upstream routers have
switched to alternate paths, the transition performed by the g-shut switched to alternate paths, the transition performed by the g-shut
initiator when the session is actually shut down, will be loopfree. initiator when the session is actually shut down, will be loopfree.
Transient forwarding loops between other routers will not be avoided Transient forwarding loops between other routers will not be avoided
with this procedure. with this procedure.
6. Dealing with Internet policies 6. Dealing with Internet policies
A side gain of the maintenance solution is that it can also reduce A side gain of the maintenance solution is that it can also reduce
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propagation of withdraw messages over eBGP sessions with shared-cost propagation of withdraw messages over eBGP sessions with shared-cost
peers and providers. peers and providers.
Proceeding like this reduces both BGP churn and traffic shifting as Proceeding like this reduces both BGP churn and traffic shifting as
routers will less likely switch to transient paths. routers will less likely switch to transient paths.
In the above example, it also prevents transient unreachabilities in In the above example, it also prevents transient unreachabilities in
the neighboring AS that are due to the sending of "abrupt" withdraw the neighboring AS that are due to the sending of "abrupt" withdraw
messages to shared-cost peers and providers. messages to shared-cost peers and providers.
7. Effect of the g-shut procedure on the convergence 7. Link Up cases
This section describes the effect of applying the solution.
7.1. Maintenance of an eBGP session
This section describes the effect of applying the solution for the
shutdown of an eBGP session.
7.1.1. Propagation on the other eBGP sessions of the g-shut initiator
Nothing is propagated on the other eBGP sessions when the out-filters
reconfiguration step is applied. The reconfiguration is indeed only
defined for its iBGP sessions.
The reconfiguration of the iBGP out-filters will trigger the
reception of alternate paths at the g-shut initiator. As the eBGP
in-filters have not been modified at that step, the old paths are
still preferred by the g-shut initiator.
7.1.2. Propagation on the other iBGP sessions of the g-shut initiator
During the out-filter reconfiguration, path updates are propagated
with a reduced local-pref value for the affected paths. As a
consequence, Route Reflectors and distant ASBRs select and propagate
alternate paths through the iBGP topology as they no longer select
the old paths as best.
When the shut-down is performed, for each affected prefix, the g-shut
initiator propagates on its iBGP sessions:
. The alternate path, if the best path was received over an eBGP
sessions.
. A withdraw, if the best path was received over an iBGP sessions.
7.1.3. Propagation of updates in an iBGP full-mesh
No transient LoC can occur if a reconfiguration of the iBGP out-
filters on the g-shut initiator is performed.
7.1.4. Propagation of updates from iBGP to iBGP in a RR hierarchy
Upon the reception of the update of a primary path with a lower
local-pref value from a client, a Route Reflector RR1 will either
propagate the update, or select an alternate path, depending on the
fact that the updated primary path is still the best one w.r.t. the
state of the Adj-Rib-In of RR1.
If the updated primary path is still the best, then the RR will
propagate an update for this path to the iBGP neighbors to which it
previously advertised the path. Hence it cannot cause transient lack
of path in the Adj-Rib-In of its iBGP neighbors.
If an alternate path is picked, and this path was also originated by
a client of RR1, an update will also be propagated to the same
neighbors as the one to which the primary path was initially
propagated. Hence it cannot cause transient lack of path in the Adj-
Rib-In of its iBGP neighbors.
If an alternate path is picked, and this path was received from a
member of its Route-Reflector iBGP full-mesh, then a withdraw message
is sent. As the alternate path has been sent over each session of
the iBGP full-mesh, the propagation of a withdraw for the primary
path of RR1 is done to routers that are expected to know the
alternate path picked by RR1.
The following example describes a situation where some corner case
timings could lead to transient unreachability from some members of
the iBGP full-mesh.
1. A Route Reflector RR1 only knew about the primary path upon
the shutdown.
2. A member of its RR full-mesh, RR2, propagates an update of
the old path with a lower local-pref.
3. Another member of its RR full-mesh, RR3 processes the
update, selects an alternate path, and propagates an update in
the mesh.
4. RR2 receives the alternate path, selects it as best, and
hence withdraws the updated old path on the iBGP sessions of the
mesh.
5. If for any reason, RR1 receives and processes the withdraw
generated in step 4 before processing the update generated in
step 3, RR1 transiently suffers from unreachability for the
affected prefix.
In such corner cases, the solution improves the iBGP convergence
behavior/LoC but does not ensure 0 packet loss, as we cannot define a
simple solution relying only on a reconfiguration of the filters of
the g-shut initiator. Improving the availability of alternate paths
in Route Reflectors, using [BestExternal], or [AddPath], seems to be
the most pragmatic solution to these corner cases.
The use of [BestExternal] in the iBGP full-mesh between RRs can solve
these corner cases by ensuring that within an AS, the advertisement
of a new path is not translated into the withdraw of a former path.
Indeed, "best-external" ensures that an ASBR does not withdraw a
previously advertised (eBGP) path when it receives an additional,
preferred path over an iBGP session. Also, "best-intra-cluster"
ensures that a RR does not withdraw a previously advertised (iBGP)
path to its non clients (e.g. other RRs in a mesh of RR) when it
receives a new, preferred path over an iBGP session.
7.2. Maintenance of an iBGP session
If the shutdown does not temper with the viability of the iBGP
topology, the described procedure is sufficient to avoid LoC.
7.3. Applicability of the g-shut procedure
The applicability of the procedure described in this draft to the
cases presented in [REQS] can be shown by combining the effects
described in this section. A complete case by case analysis will be
provided in the next versions of the draft.
7.4. Summary of operations
This section summarizes the configurations and actions to be
performed to support the g-shut procedure for eBGP peering links.
7.4.1. Pre-configuration
On each ASBR supporting the g-shut procedure, set-up an out-filter
applied on all iBGP sessions of the ASBR, that :
. sets the local-pref of the paths tagged with the g-shut community
to a low value
. removes the g-shut community from the path.
7.4.2. Operations at maintenance time
On the g-shut initiator :
. Apply an in-filter on the maintained BGP session to tag the paths
received over the session with the g-shut community.
. Apply an out-filter on the maintained BGP session to tag the paths
propagated over the session with the g-shut community.
. Wait for convergence to happen.
. Perform a BGP session shutdown.
8. Link Up cases
We identify two potential causes for transient packet losses upon an We identify two potential causes for transient packet losses upon an
eBGP link up event. The first one is local to the g-shut initiator, eBGP link up event. The first one is local to the g-no-shut
the second one is due to the BGP convergence following the injection initiator, the second one is due to the BGP convergence following the
of new best paths within the iBGP topology. injection of new best paths within the iBGP topology.
8.1. Unreachability local to the ASBR 7.1. Unreachability local to the ASBR
An ASBR that selects as best a path received over a newly brought up An ASBR that selects as best a path received over a newly brought up
eBGP session may transiently drop traffic. This can typically happen eBGP session may transiently drop traffic. This can typically happen
when the nexthop attribute differs from the IP address of the eBGP when the nexthop attribute differs from the IP address of the eBGP
peer, and the receiving ASBR has not yet resolved the MAC address peer, and the receiving ASBR has not yet resolved the MAC address
associated with the IP address of that "third party" nexthop. associated with the IP address of that "third party" nexthop.
A BGP speaker implementation could avoid such losses by ensuring that A BGP speaker implementation could avoid such losses by ensuring that
"third party" nexthops are resolved before installing paths using "third party" nexthops are resolved before installing paths using
these in the RIB. these in the RIB.
If the link up event corresponds to an eBGP session that is being If the link up event corresponds to an eBGP session that is being
manually brought up, over an already up multi-access link, then the manually brought up, over an already up multi-access link, then the
operator can ping third party nexthops that are expected to be used operator can ping third party nexthops that are expected to be used
before actually bringing the session up, or ping directed broadcast before actually bringing the session up, or ping directed broadcast
the subnet IP address of the link. By proceeding like this, the MAC the subnet IP address of the link. By proceeding like this, the MAC
addresses associated with these third party nexthops will be resolved addresses associated with these third party nexthops will be resolved
by the g-no-shut initiator. by the g-no-shut initiator.
8.2. iBGP convergence 7.2. iBGP convergence
Similar corner cases as described in Section 7.1.4 for the link down Similar corner cases as described in Appendix C.1.4 for the link down
case, can occur during an eBGP link up event. case, can occur during an eBGP link up event.
A typical example for such transient unreachability for a given A typical example for such transient unreachability for a given
prefix is the following : prefix is the following :
1. A Route Reflector, RR1, is initially advertising the current 1. A Route Reflector, RR1, is initially advertising the current
best path to the members of its iBGP RR full-mesh. It best path to the members of its iBGP RR full-mesh. It
propagated that path within its RR full-mesh. Another route propagated that path within its RR full-mesh. Another route
reflector of the full-mesh, RR2, knows only that path towards reflector of the full-mesh, RR2, knows only that path towards
the prefix. the prefix.
skipping to change at page 15, line 9 skipping to change at page 12, line 25
advertisement of a new route is not translated into the withdraw of a advertisement of a new route is not translated into the withdraw of a
former route. former route.
Indeed, "best-external" ensures that an ASBR does not withdraw a Indeed, "best-external" ensures that an ASBR does not withdraw a
previously advertised (eBGP) path when it receives an additional, previously advertised (eBGP) path when it receives an additional,
preferred path over an iBGP session. Also, "best-intra-cluster" preferred path over an iBGP session. Also, "best-intra-cluster"
ensures that a RR does not withdraw a previously advertised (iBGP) ensures that a RR does not withdraw a previously advertised (iBGP)
path to its non clients (e.g. other RRs in a mesh of RR) when it path to its non clients (e.g. other RRs in a mesh of RR) when it
receives a new, preferred path over an iBGP session. receives a new, preferred path over an iBGP session.
9. Alternative techniques with limited applicability 8. IANA considerations
Applying the g-shut procedure is rendered much easier with a reserved
g-shut community value. The community value 0xFFFF0000 has been
reserved from the FCFS community pool for this purpose.
9. Security Considerations
By providing the g-shut service to a neighboring AS, an ISP provides
means to this neighbor to lower the local-pref value assigned to the
paths received from this neighbor.
The neighbor could abuse the technique and do inbound traffic
engineering by declaring some prefixes as undergoing a maintenance so
as to switch traffic to another peering link.
If this behavior is not tolerated by the ISP, it SHOULD monitor the
use of the g-shut community by this neighbor.
ASes which support the g-shut procedure SHOULD remove the community
value(s) that they use for g-shut from the paths received from
neighboring ASes that do not support the procedure or to whom the
service is not provided. Doing so prevents malignant ASes from using
the community through intermediate ASes that do not support the
feature, in order to perform inbound traffic engineering.
10. Acknowledgments
The authors wish to thank Olivier Bonaventure and Pradosh Mohapatra
for their useful comments on this work.
11. References
[AddPath] D. Walton, A. Retana, and E. Chen, "Advertisement of
Multiple Paths in BGP", draft-walton-bgp-add-paths-06.txt
(work in progress).
[BestExternal]
Marques, P., Fernando, R., Chen, E., and P. Mohapatra,
"Advertisement of the best-external route to IBGP",
draft-ietf-idr-best-external-00.txt, May 2009.
[REQS] Decraene, B., Francois, P., Pelsser, C., Ahmad, Z., and T.
Takeda, "Requirements for the graceful shutdown of BGP
sessions",
draft-ietf-grow-bgp-graceful-shutdown-requirements-
01.txt, October 2009.
[RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
Communities Attribute", RFC 4360, February 2006.
[Clarification4360]
Decraene, B., Vanbever, L., and P. Francois, "RFC 4360
Clarification Request",
draft-decraene-idr-rfc4360-clarification-00,
October 2009.
[BGPWKC] "http://www.iana.org/assignments/
bgp-well-known-communities".
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Appendix A. Summary of operations
This section summarizes the configurations and actions to be
performed to support the g-shut procedure for eBGP peering links.
A.1. Pre-configuration
On each ASBR supporting the g-shut procedure, set-up an out-filter
applied on all iBGP sessions of the ASBR, that :
. sets the local-pref of the paths tagged with the g-shut community
to a low value
. removes the g-shut community from the path.
Optionally, add an AS specific g-shut community on the path to
indicate that this path is to be shutdown. If some ingress ASBRs
reset the local preference attribute, this AS specific g-shut
community will be used to override other local preference changes.
A.2. Operations at maintenance time
On the g-shut initiator :
. Apply an in-filter on the maintained eBGP session to tag the paths
received over the session with the g-shut community.
. Apply an out-filter on the maintained eBGP session to tag the
paths propagated over the session with the g-shut community.
. Wait for convergence to happen.
. Perform a BGP session shutdown.
Appendix B. Alternative techniques with limited applicability
A few alternative techniques have been considered to provide g-shut A few alternative techniques have been considered to provide g-shut
capabilities but have been rejected due to their limited capabilities but have been rejected due to their limited
applicability. This section describe them for possible reference. applicability. This section describe them for possible reference.
9.1. In-filter reconfiguration B.1. In-filter reconfiguration
An In-filter reconfiguration on the eBGP session undergoing the An In-filter reconfiguration on the eBGP session undergoing the
maintenance could be performed instead of out-filter reconfigurations maintenance could be performed instead of out-filter reconfigurations
on the iBGP sessions of the g-shut initiator. on the iBGP sessions of the g-shut initiator.
Upon the application of the maintenance procedure, if the g-shut Upon the application of the maintenance procedure, if the g-shut
initiator has an alternate path in its Adj-Rib-In, it will switch to initiator has an alternate path in its Adj-Rib-In, it will switch to
it directly. it directly.
If this new path was advertised by an eBGP neighbor of the g-shut If this new path was advertised by an eBGP neighbor of the g-shut
skipping to change at page 16, line 21 skipping to change at page 15, line 40
path first, the withdraw from ASBR2 does not trigger unreachability path first, the withdraw from ASBR2 does not trigger unreachability
in other nodes, as the old path is still available. Indeed, even in other nodes, as the old path is still available. Indeed, even
though it receives alternate paths, the g-shut initiator keeps using though it receives alternate paths, the g-shut initiator keeps using
its old path as best as the in-filter of the maintained eBGP session its old path as best as the in-filter of the maintained eBGP session
has not been modified yet. has not been modified yet.
Applying the out-filter reconfiguration also prevents packet loops Applying the out-filter reconfiguration also prevents packet loops
between the g-shut initiator and its direct neighbors when between the g-shut initiator and its direct neighbors when
encapsulation is not used between the ASBRs of the AS. encapsulation is not used between the ASBRs of the AS.
9.2. Multi Exit Discriminator tweaking B.2. Multi Exit Discriminator tweaking
The MED attribute of the paths to be avoided can be increased so as The MED attribute of the paths to be avoided can be increased so as
to force the routers in the neighboring AS to select other paths. to force the routers in the neighboring AS to select other paths.
The solution only works if the alternate paths are as good as the The solution only works if the alternate paths are as good as the
initial ones with respect to the Local-Pref value and the AS Path initial ones with respect to the Local-Pref value and the AS Path
Length value. In the other cases, increasing the MED value will not Length value. In the other cases, increasing the MED value will not
have an impact on the decision process of the routers in the have an impact on the decision process of the routers in the
neighboring AS. neighboring AS.
9.3. IGP distance Poisoning B.3. IGP distance Poisoning
The distance to the BGP nexthop corresponding to the maintained The distance to the BGP nexthop corresponding to the maintained
session can be increased in the IGP so that the old paths will be session can be increased in the IGP so that the old paths will be
less preferred during the application of the IGP distance tie-break less preferred during the application of the IGP distance tie-break
rule. However, this solution only works for the paths whose rule. However, this solution only works for the paths whose
alternates are as good as the old paths with respect to their Local- alternates are as good as the old paths with respect to their Local-
Pref value, their AS Path length, and their MED value. Pref value, their AS Path length, and their MED value.
Also, this poisoning cannot be applied when nexthop self is used as Also, this poisoning cannot be applied when nexthop self is used as
there is no nexthop specific to the maintained session to poison in there is no nexthop specific to the maintained session to poison in
the IGP. the IGP.
10. IANA considerations Appendix C. Effect of the g-shut procedure on the convergence
Applying the g-shut procedure is rendered much easier with a reserved This section describes the effect of applying the solution.
g-shut community value. Hence this draft suggests to reserve a
community value, e.g., 0xFFFF0000, for this purpose.
11. Security Considerations C.1. Maintenance of an eBGP session
By providing the g-shut service to a neighboring AS, an ISP provides This section describes the effect of applying the solution for the
means to this neighbor to lower the local-pref value assigned to the shutdown of an eBGP session.
paths received from this neighbor.
The neighbor could abuse the technique and do inbound traffic C.1.1. Propagation on the other eBGP sessions of the g-shut initiator
engineering by declaring some prefixes as undergoing a maintenance so
as to switch traffic to another peering link.
If this behavior is not tolerated by the ISP, it SHOULD monitor the Nothing is propagated on the other eBGP sessions when the out-filters
use of the g-shut community by this neighbor. reconfiguration step is applied. The reconfiguration is indeed only
defined for its iBGP sessions.
12. Acknowledgments The reconfiguration of the iBGP out-filters will trigger the
reception of alternate paths at the g-shut initiator. As the eBGP
in-filters have not been modified at that step, the old paths are
still preferred by the g-shut initiator.
The authors wish to thank Olivier Bonaventure and Pradosh Mohapatra C.1.2. Propagation on the other iBGP sessions of the g-shut initiator
for their useful comments on this work.
13. References During the out-filter reconfiguration, path updates are propagated
with a reduced local-pref value for the affected paths. As a
consequence, Route Reflectors and distant ASBRs select and propagate
alternate paths through the iBGP topology as they no longer select
the old paths as best.
[AddPath] D. Walton, A. Retana, and E. Chen, "Advertisement of When the shut-down is performed, for each affected prefix, the g-shut
Multiple Paths in BGP", draft-walton-bgp-add-paths-06.txt initiator propagates on its iBGP sessions:
(work in progress).
[BestExternal] . The alternate path, if the best path was received over an eBGP
Marques, P., Fernando, R., Chen, E., and P. Mohapatra, sessions.
"Advertisement of the best-external route to IBGP",
draft-marques-idr-best-external-00.txt, July 2008.
[REQS] Decraene, B., Francois, P., Pelsser, C., and Z. Ahmad, . A withdraw, if the best path was received over an iBGP sessions.
"Requirements for the graceful shutdown of BGP sessions",
draft-ietf-grow-bgp-graceful-shutdown-requirements-00.txt C.1.3. Propagation of updates in an iBGP full-mesh
, May 2009.
No transient LoC can occur if a reconfiguration of the iBGP out-
filters on the g-shut initiator is performed.
C.1.4. Propagation of updates from iBGP to iBGP in a RR hierarchy
Upon the reception of the update of a primary path with a lower
local-pref value from a client, a Route Reflector RR1 will either
propagate the update, or select an alternate path, depending on the
fact that the updated primary path is still the best one w.r.t. the
state of the Adj-Rib-In of RR1.
If the updated primary path is still the best, then the RR will
propagate an update for this path to the iBGP neighbors to which it
previously advertised the path. Hence it cannot cause transient lack
of path in the Adj-Rib-In of its iBGP neighbors.
If an alternate path is picked, and this path was also originated by
a client of RR1, an update will also be propagated to the same
neighbors as the one to which the primary path was initially
propagated. Hence it cannot cause transient lack of path in the Adj-
Rib-In of its iBGP neighbors.
If an alternate path is picked, and this path was received from a
member of its Route-Reflector iBGP full-mesh, then a withdraw message
is sent. As the alternate path has been sent over each session of
the iBGP full-mesh, the propagation of a withdraw for the primary
path of RR1 is done to routers that are expected to know the
alternate path picked by RR1.
The following example describes a situation where some corner case
timings could lead to transient unreachability from some members of
the iBGP full-mesh.
1. A Route Reflector RR1 only knew about the primary path upon
the shutdown.
2. A member of its RR full-mesh, RR2, propagates an update of
the old path with a lower local-pref.
3. Another member of its RR full-mesh, RR3 processes the
update, selects an alternate path, and propagates an update in
the mesh.
4. RR2 receives the alternate path, selects it as best, and
hence withdraws the updated old path on the iBGP sessions of the
mesh.
5. If for any reason, RR1 receives and processes the withdraw
generated in step 4 before processing the update generated in
step 3, RR1 transiently suffers from unreachability for the
affected prefix.
In such corner cases, the solution improves the iBGP convergence
behavior/LoC but does not ensure 0 packet loss, as we cannot define a
simple solution relying only on a reconfiguration of the filters of
the g-shut initiator. Improving the availability of alternate paths
in Route Reflectors, using [BestExternal], or [AddPath], seems to be
the most pragmatic solution to these corner cases.
The use of [BestExternal] in the iBGP full-mesh between RRs can solve
these corner cases by ensuring that within an AS, the advertisement
of a new path is not translated into the withdraw of a former path.
Indeed, "best-external" ensures that an ASBR does not withdraw a
previously advertised (eBGP) path when it receives an additional,
preferred path over an iBGP session. Also, "best-intra-cluster"
ensures that a RR does not withdraw a previously advertised (iBGP)
path to its non clients (e.g. other RRs in a mesh of RR) when it
receives a new, preferred path over an iBGP session.
C.2. Maintenance of an iBGP session
If the shutdown does not temper with the viability of the iBGP
topology, the described procedure is sufficient to avoid LoC.
Authors' Addresses Authors' Addresses
Pierre Francois Pierre Francois
Universite catholique de Louvain Universite catholique de Louvain
Place Ste Barbe, 2 Place Ste Barbe, 2
Louvain-la-Neuve 1348 Louvain-la-Neuve 1348
BE BE
Email: pierre.francois@uclouvain.be Email: pierre.francois@uclouvain.be
URI: http://inl.info.ucl.ac.be/pfr URI: http://inl.info.ucl.ac.be/pfr
Bruno Decraene Bruno Decraene
France Telecom France Telecom
38-40 rue du General Leclerc 38-40 rue du General Leclerc
92794 Issi Moulineaux cedex 9 92794 Issi Moulineaux cedex 9
FR FR
Email: bruno.decraene@orange-ftgroup.com Email: bruno.decraene@orange-ftgroup.com
Cristel Pelsser Cristel Pelsser
NTT Corporation Internet Initiative Japan
9-11, Midori-Cho 3 Chrome Jinbocho Mitsui Bldg.
Musashino-Shi, Tokyo 180-8585 1-105 Kanda Jinbo-cho
Tokyo 101-0051
JP JP
Email: pelsser.cristel@lab.ntt.co.jp Email: pelsser.cristel@iij.ad.jp
Clarence Filsfils Clarence Filsfils
Cisco Systems Cisco Systems
De kleetlaan 6a De kleetlaan 6a
Diegem 1831 Diegem 1831
BE BE
Email: cfilsfil@cisco.com Email: cfilsfil@cisco.com
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