Individual Contributor

pfrpfr@gmail.com

Orange

bruno.decraene@orange.com

Strasbourg University

pelsser@unistra.fr

Arrcus, Inc.

keyur@arrcus.com

Cisco Systems

cfilsfil@cisco.com

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. It defines a well-known BGP community, called GRACEFUL_SHUTDOWN, to signal the graceful shutdown of paths.

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 at best, without changing the BGP protocol. This document defines a well-known community , called GRACEFUL_SHUTDOWN, for the purpose of reducing the management overhead of gracefully shutting down BGP sessions. The well-known community allows implementers to provide an automated 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 .

graceful shutdown initiator: a router on which the session shutdown is performed for the maintenance. graceful shutdown receiver: a router that has a BGP session, to be shutdown, with the graceful shutdown initiator. Initiator AS: the Autonomous System of the graceful shutdown initiator. Receiver AS: the Autonomous System of the graceful shutdown receiver. Loss of Connectivity (LoC: the state when a router has no path toward an affected prefix.

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 toward 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 toward 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.

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

All solutions that increase the availability of alternate BGP paths at routers performing packet lookups in BGP tables such as and 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 graceful shutdown 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 receiver 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.

The goal of this procedure is to retain the paths to be shutdown between the peers, but with a lower LOCAL_PREF value, allowing the paths to remain in use while alternate paths are selected and propagated, rather than simply withdrawing the paths. describes configurations and actions to be performed for the graceful shutdown of BGP sessions.

The graceful shutdown 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.

This section describes configurations and actions to be performed for the graceful shutdown of EBGP peering links.

On each ASBR supporting the graceful shutdown receiver procedure, an inbound BGP route policy is applied on all EBGP sessions of the ASBR, that: matches the GRACEFUL_SHUTDOWN community sets the LOCAL_PREF attribute of the paths tagged with the GRACEFUL_SHUTDOWN community to a low value Note that in the case where an AS is aggregating multiple routes under a covering prefix, it is recommended to filter out the GRACEFUL_SHUTDOWN community from the resulting aggregate BGP route. By doing so, the setting of the GRACEFUL_SHUTDOWN 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 graceful shutdown behavior.

On the graceful shutdown initiator, upon maintenance time, it is required to: apply an outbound BGP route policy on the EBGP session to be shutdown. This policy tags the paths propagated over the session with the GRACEFUL_SHUTDOWN community. This will trigger the BGP implementation to re-advertise all active routes previously advertised, and tag them with the GRACEFUL_SHUTDOWN community. apply an inbound BGP route policy on the maintained EBGP session to tag the paths received over the session with the GRACEFUL_SHUTDOWN community. wait for convergence to happen. shutdown the EBGP session, optionally using to communicate the reason of the shutdown. In the case of a shutdown of the whole router, in addition to the graceful shutdown of all EBGP sessions, there is a need to graceful shutdown the routes originated by this router (e.g, BGP aggregates redistributed from other protocols, including static routes). This can be performed by tagging such routes with the GRACEFUL_SHUTDOWN community.

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 graceful shutdown feature SHOULD: Update all the paths propagated over the corresponding EBGP session, tagging the GRACEFUL_SHUTDOWN community to them. Any subsequent update sent over the session being gracefully shut down would be tagged with the GRACEFUL_SHUTDOWN community. Lower the LOCAL_PREF value of the paths received over the EBGP session being shut down. Optionally shut down the session after a configured time. Prevent the GRACEFUL_SHUTDOWN 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 graceful shutdown feature SHOULD also automatically install the BGP policies that are supposed to be configured, as described in for sessions over which graceful shutdown is to be supported.

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 graceful shutdown IBGP session, then the shutdown of an IBGP session does not lead to transient unreachability. As a consequence, no specific graceful shutdown action is required.

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

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 NEXT_HOP 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" NEXT_HOP. A BGP speaker implementation could avoid such losses by ensuring that "third party" NEXT_HOPs 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 NEXT_HOP 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 NEXT_HOP will be resolved by the graceful no-shut initiator.

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. 1. RR1 is initially advertising the current best path to the members of its IBGP RR full-mesh. It propagated that path within its RR full-mesh. RR2 knows only that path toward the prefix. 2. RR3 receives a new best path originated by the "graceful no-shut" initiator, being one of its RR clients. RR3 selects it as best, and propagates an UPDATE within its RR full-mesh, i.e., to RR1 and RR2. 3. RR1 receives that path, reruns its decision process, and picks this new path as best. As a result, RR1 withdraws its previously announced best-path on the IBGP sessions of its RR full-mesh. 4. If, for any reason, RR3 processes the withdraw generated in step 3, before processing the update generated in step 2, RR3 transiently suffers from unreachability for the affected prefix. The use of 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.

The IANA has assigned the community value 0xFFFF0000 to the planned-shut community in the "BGP Well-known Communities" registry. IANA is requested to change the name planned-shut to GRACEFUL_SHUTDOWN and set this document as the reference.

By providing the graceful shutdown 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 graceful shutdown community by this neighbor.

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

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

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.

The distance to the BGP NEXT_HOP 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.

This appendix is non-normative. Example routing policy configurations to honor the GRACEFUL_SHUTDOWN well-known BGP community.