BGP4 Graceful Shutdown

BGP4 graceful shutdown automatically reroutes traffic to an alternate path before shutting down a neighbor. Graceful shutdown minimizes traffic loss during a planned shutdown and re-establishment of BGP4 sessions.

BGP protocol is heavily used in the data center solutions where, for resiliency purposes, redundant spine and leaf devices are deployed to reduce the consequence of breakdowns.

In the context of a planned maintenance operation, such as firmware upgrade, software upgrade, line card replacement, SFP replacement, cable replacement or node replacement, you may need to shut down BGP peer links or a complete switch. This action results in traffic disruption until the underlay network converges, even when alternate paths are available.

By configuring BGP graceful shutdown when an alternate path is available, you can reroute traffic on the backup path before the BGP sessions are torn down (Make before Break). BGP graceful shutdown is an automated procedure that reroutes traffic to the alternate path before shutting down the neighbor. The procedure uses BGP attributes such as LOCAL_PREFERENCE, AS_PATH PREPEND and GRACEFUL_SHUTDOWN community to make a route less preferred.

Note

You can override these parameters by using a route map.

When BGP graceful shutdown is initiated on one or more BGP neighbors, BGP starts a graceful shutdown timer on the graceful shutdown initiator and then refreshes routes to all graceful shutdown neighbors with updated BGP attributes, such as LOCAL_PREFERENCE for confederations or iBGP, or AS_PATH PREPEND for eBGP. The best path calculation on all graceful shutdown neighbors learned routes is then triggered, making the routes by way of the graceful shutdown initiator less preferable, so that the traffic from graceful shutdown neighbors is re-routed to alternate paths.

However, the path by way of the graceful shutdown initiator is still valid until the graceful shutdown timer expires. After the graceful shutdown timer expires, all the graceful shutdown neighbors are shut down on the graceful shutdown initiator and all traffic is rerouted to alternate paths. You can now carry the maintenance operation.

Note

You should wait for the timer to expire before carrying out a maintenance operation. When the graceful shutdown procedure is deactivated while the timer is running, the route refresh using the graceful shutdown parameters aborts, a new route refresh is triggered, and the graceful shutdown timer stops.

When the graceful shutdown initiator has an outbound policy associated with a particular graceful shutdown neighbor, the outbound policy is applied before the graceful shutdown parameters are applied.

When the graceful shutdown neighbor has an inbound policy associated with the graceful shutdown initiator that is modifying the LOCAL_PREFERENCE or AS_PATH attributes, the graceful shutdown parameters become irrelevant. As a best practice, modify the existing inbound policy rule to match the graceful shutdown community attribute and to make the route less preferred. The send community attribute should also be negotiated between the graceful shutdown initiator and the neighbor.

BGP4 graceful shutdown supports high availability (HA). During a restart, the peer is instantly shut down and the graceful shutdown timer is not started.

Note

The graceful shutdown community attribute should not be considered for aggregation. To prevent the graceful shutdown behavior being applied to the entire BGP aggregate route when an autonomous system (AS) aggregates multiple routes under a cover prefix, the graceful shutdown community should be filtered out of the aggregate route by using the no match community command.

This implementation of BGP4 Graceful Shutdown conforms to RFC 6198 and Graceful BGP session shutdown draft-ietf-grow-bgp-gshut-03.