Network Working Group Pierre Francois
Internet-Draft Individual Contributor
Intended status: Informational B. Decraene
Expires: December 24, 2017 Orange
C. Pelsser
Strasbourg University
K. Patel
Arrcus, Inc.
C. Filsfils
Cisco Systems
June 22, 2017

Graceful BGP session shutdown


This draft describes operational procedures aimed at reducing the amount of traffic lost during planned maintenances of routers or links, involving the shutdown of BGP peering sessions.

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Table of Contents

1. Introduction

Routing changes in BGP can be caused by planned, maintenance operations. This document discusses operational procedures to be applied in order to reduce or eliminate losses of packets during the maintenance. These losses come from the transient lack of reachability during the BGP convergence following the shutdown of an eBGP peering session between two Autonomous System Border Routers (ASBR).

This document presents procedures for the cases where the forwarding plane is impacted by the maintenance, hence when the use of Graceful Restart does not apply.

The procedures described in this document can be applied to reduce or avoid packet loss for outbound and inbound traffic flows initially forwarded along the peering link to be shut down. These procedures trigger, in both involved ASes, rerouting to the alternate path, while allowing routers to keep using old paths until alternate ones are learned, installed in the RIB and in the FIB. This ensures that routers always have a valid route available during the convergence process.

The goal of the document is to meet the requirements described in [RFC6198] at best, without changing the BGP protocol.

Still, it explains why reserving a community value for the purpose of BGP session graceful shutdown would reduce the management overhead bound with the solution. It would also allow vendors to provide an automatic graceful shutdown mechanism that does not require any 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

g-shut initiator: a router on which the session shutdown is performed for the maintenance.

g-shut neighbor: a router that peers with the g-shut initiator via (one of) the session(s) to be shut down.

Initiator AS: the Autonomous System of the g-shut initiator.

Neighbor AS: the Autonomous System of the g-shut neighbor.

Loss of Connectivity (LoC: the state when a router has no path towards an affected prefix.

3. Packet loss upon manual eBGP session shutdown

Packets can be lost during a manual shutdown of an eBGP session for two reasons.

First, routers involved in the convergence process can transiently lack of paths towards an affected prefix, and drop traffic destined to this prefix. This is because alternate paths can be hidden by nodes of an AS. This happens when the paths are not selected as best by the ASBR that receive them on an eBGP session, or by Route Reflectors that do not propagate them further in the iBGP topology because they do not select them as best.

Second, within the AS, the FIB of routers can be transiently inconsistent during the BGP convergence and packets towards affected prefixes can loop and be dropped. Note that these loops only happen when ASBR-to-ASBR encapsulation is not used within the AS.

This document only addresses the first reason.

4. Practices to avoid packet losses

This section describes means for an ISP to reduce the transient loss of packets upon a manual shutdown of a BGP session.

4.1. Improving availability of alternate paths

All solutions that increase the availability of alternate BGP paths at routers performing packet lookups in BGP tables such as [I-D.ietf-idr-best-external] and [RFC7911] help in reducing the LoC bound with manual shutdown of eBGP sessions.

One of such solutions increasing diversity in such a way that, at any single step of the convergence process following the eBGP session shutdown, a BGP router does not receive a message withdrawing the only path it currently knows for a given NLRI, allows for a simplified g-shut procedure.

Note that the LoC for the inbound traffic of the maintained router, induced by a lack of alternate path propagation within the iBGP topology of a neighboring AS is not under the control of the operator performing the maintenance. The part of the procedure aimed at avoiding LoC for incoming paths can thus be applied even if no LoC are expected for the outgoing paths.

4.2. Make before break convergence: g-shut

This section describes configurations and actions to be performed for the graceful shutdown of eBGP sessions, iBGP sessions or a whole BGP speaker.

The goal of this procedure is to let, in both ASes, the paths being shutdown visible, but with a lower LOCAL_PREF value, while alternate paths spread through the iBGP topology. Instead of withdrawing the path, routers of an AS will keep on using it until they become aware of alternate paths.

4.2.1. eBGP g-shut

This section describes configurations and actions to be performed for the graceful shutdown of eBGP peering links. Pre-configuration

On each ASBR supporting the g-shut procedure, an outbound BGP route policy is applied on all iBGP sessions of the ASBR, that:

Note that in the case where an AS is aggregating multiple routes under a covering prefix, it is recommended to filter out the g-shut community from the resulting aggregate BGP route. By doing so, the setting of the g-shut community on one of the aggregated routes will not let the entire aggregate inherit the community. Not doing so would let the entire aggregate undergo the g-shut behavior. Operations at maintenance time

On the g-shut initiator, upon maintenance time, it is required to: BGP implementation support for g-Shut

A BGP router implementation MAY provide features aimed at automating the application of the graceful shutdown procedures described above.

Upon a session shutdown specified as graceful by the operator, a BGP implementation supporting a g-shut feature SHOULD:

  1. On the eBGP side, update all the paths propagated over the corresponding eBGP session, tagging the g-shut community to them. Any subsequent update sent to the session being gracefully shut down would be tagged with the g-shut community.
  2. On the iBGP side, lower the LOCAL_PREF value of the paths received over the eBGP session being shut down, upon their propagation over iBGP sessions. Optionally, also tag these paths with an AS specific g-shut community.
  3. Optionally shut down the session after a configured time.
  4. Prevent the g-shut community from being inherited by a path that would aggregate some paths tagged with the GSHUT community. This behavior avoids the GSHUT procedure to be applied to the aggregate upon the graceful shutdown of one of its covered prefixes.

A BGP implementation supporting a g-shut feature SHOULD also automatically install the BGP policies that are supposed to be configured, as described in Section for sessions over which g-shut is to be supported.

4.2.2. iBGP g-shut

For the shutdown of an iBGP session, provided 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 all prefixes advertised on this g-shut iBGP session, then the shutdown of an iBGP session does not lead to transient unreachability. As a consequence, no specific g-shut action is required.

4.2.3. Router g-shut

In the case of a shutdown of a router, a reconfiguration of the outbound BGP route policies of the g-shut initiator SHOULD be performed to set a low LOCAL_PREF value for the paths originated by the g-shut initiator (e.g, BGP aggregates redistributed from other protocols, including static routes).

This behavior is equivalent to the recommended behavior for paths "redistributed" from eBGP sessions to iBGP sessions in the case of the shutdown of an ASBR.

5. Forwarding modes and transient forwarding loops during convergence

The g-shut procedure or the solutions improving the availability of alternate paths, do not change the fact that BGP convergence and the subsequent FIB updates are run independently on each router of the ASes. If the AS applying the solution does not rely on encapsulation to forward packets from the Ingress Border Router to the Egress Border Router, then transient forwarding loops and consequent packet losses can occur during the convergence process. If zero LoC is required, encapsulation is required between ASBRs of the AS.

6. Link Up cases

We identify two potential causes for transient packet losses upon an eBGP link up event. The first one is local to the g-no-shut initiator, the second one is due to the BGP convergence following the injection of new best paths within the iBGP topology.

6.1. Unreachability local to the ASBR

An ASBR that selects as best a path received over a newly brought up eBGP session may transiently drop traffic. This can typically happen when the nexthop attribute differs from the IP address of the eBGP peer, and the receiving ASBR has not yet resolved the MAC address associated with the IP address of that "third party" nexthop.

A BGP speaker implementation could avoid such losses by ensuring that "third party" nexthops are resolved before installing paths using these in the RIB.

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 operator can ping third party nexthops that are expected to be used before actually bringing the session up, or ping directed broadcast the subnet IP address of the link. By proceeding like this, the MAC addresses associated with these third party nexthops will be resolved by the g-no-shut initiator.

6.2. iBGP convergence

Corner cases leading to LoC can occur during an eBGP link up event.

A typical example for such transient unreachability for a given prefix is the following:

Let's consider 3 route reflectors RR1, RR2, RR3. There is a full mesh of iBGP session between them.

The use of [I-D.ietf-idr-best-external] among the RR of the iBGP full-mesh can solve these corner cases by ensuring that within an AS, the advertisement of a new route is not translated into the withdraw of a former route.

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. IANA assigned g-shut BGP community

Applying the g-shut procedure is rendered much easier with the use of a single g-shut BGP community value [RFC1997] which could be used on all eBGP sessions, for both inbound and outbound signaling. The community value 0xFFFF0000 has been assigned by IANA for this purpose.

8. IANA Considerations

This document has no actions for IANA.

9. Security Considerations

By providing the g-shut service to a neighboring AS, an ISP provides means to this neighbor and possibly its downstream ASes 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.

10. Acknowledgments

The authors wish to thank Olivier Bonaventure, Pradosh Mohapatra and Job Snijders for their useful comments on this work.

11. References

11.1. Normative References

[RFC1997] Chandra, R., Traina, P. and T. Li, "BGP Communities Attribute", RFC 1997, DOI 10.17487/RFC1997, August 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC6198] Decraene, B., Francois, P., Pelsser, C., Ahmad, Z., Elizondo Armengol, A. and T. Takeda, "Requirements for the Graceful Shutdown of BGP Sessions", RFC 6198, DOI 10.17487/RFC6198, April 2011.

11.2. Informative References

[I-D.ietf-idr-best-external] Marques, P., Fernando, R., Chen, E., Mohapatra, P. and H. Gredler, "Advertisement of the best external route in BGP", Internet-Draft draft-ietf-idr-best-external-05, January 2012.
[RFC7911] Walton, D., Retana, A., Chen, E. and J. Scudder, "Advertisement of Multiple Paths in BGP", RFC 7911, DOI 10.17487/RFC7911, July 2016.

Appendix A. Alternative techniques with limited applicability

A few alternative techniques have been considered to provide g-shut capabilities but have been rejected due to their limited applicability. This section describe them for possible reference.

A.1. Multi Exit Discriminator tweaking

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.

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 Length value. In the other cases, increasing the MED value will not have an impact on the decision process of the routers in the neighboring AS.

A.2. IGP distance Poisoning

The distance to the BGP nexthop corresponding to the maintained 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 rule. However, this solution only works for the paths whose alternates are as good as the old paths with respect to their Local-Pref value, their AS Path length, and their MED value.

Also, this poisoning cannot be applied when nexthop self is used as there is no nexthop specific to the maintained session to poison in the IGP.

Authors' Addresses

Pierre Francois Individual Contributor EMail:
Bruno Decraene Orange EMail:
Cristel Pelsser Strasbourg University EMail:
Keyur Patel Arrcus, Inc. EMail:
Clarence Filsfils Cisco Systems EMail: