Graceful restart scenarios

Graceful restart includes the following scenarios:

• Re-advertise label to its upstream neighbors—When the restarting router, acting as a transit LSR, can recover a FEC based on the Label Mapping it receives from its GR helper, and the local forwarding state successfully, it re-advertises the same label to all of its upstream neighbors.

  As part of supporting GR, the Label Management component also makes sure that those labels that are used to advertise to upstream neighbors before GR happens is not re-used for the new LSP coming up while GR is in-progress. However, when the previously used label is released because the LSP has gone down during GR, the label can be re-allocated for the new LSP.

• Clearing MPLS LDP neighbors— For the clear mpls ldp neighbor command, the configured reconnect and recovery timer values is sent to the peer when both are configured with LDP graceful restart. Note that in this scenario, both routers are acting in helper-only mode. Therefore, after the session comes back up, both routers exchange their bindings and go through the recovery procedure. There is no traffic loss when the reconnect and the recovery timers do not time out.

  On a router configured for helper-only mode, the clearing mpls ldp neighbor command results in immediate reconnect timeout at the remote end. Therefore, in this scenario all bindings at the remote end associated with the session are deleted due to reconnect time out. On the local node the recovery timer of zero results in immediate clearing of the forwarding entries.