BGP graceful restart (GR)

When a BGP speaker shuts down, planned or unplanned, the routes that are advertised by the speaker and reachable via the speaker now become unreachable. Upon detecting that the BGP speaker has restarted, the peers delete the routes and re-add them when the restarting router advertises them again. This results in route-flap across the BGP connectivity and impacts multiple routing domains causing transient instability in the network.

The Graceful Restart capability is supported as a ‘helper router’ on the switches. In 'helper only' mode the router helps the other restarting router by holding the received routes from it as stale routes and not dropping them.

  1. To establish a BGP session with a peer, a BGP GR Restarter sends an OPEN message with GR capability to the peer.

  2. Upon receipt of this message, the peer is aware that the sending router is capable of Graceful Restart, and sends an OPEN message with GR Capability to the GR Restarter to establish a GR session. If neither party has the GR capability, the session established between them will not be GR capable.

  3. The GR session between the GR Restarter and its peer goes down when the GR Restarter restarts BGP. The GR capable peer will mark all routes associated with the GR Restarter as stale. However, during the configured GR Time, it still uses these routes for packet forwarding.

  4. After the restart, the GR Restarter will reestablish a GR session with its peer and send a new GR message notifying the completion of restart. Routing information is exchanged between them for the GR Restarter to create a new routing table and forwarding table with stale routing information removed. Then the BGP routing convergence is complete.