8.11.2 Graceful restart procedure

Graceful restart is a function that maintains communication for relaying the Switch in conjunction with a neighboring switch when the master switch is switched during stack configuration or when the unicast routing program is restarted by an operation command. The following shows a specific implementation of graceful restart.

• Provides the neighboring router with a facility to assist in graceful restart. Adds the ability to recognize that the previous connection is continuing, rather than disconnecting and reconnecting when a graceful restart connection request is received. This prevents the connection with the neighboring router from being disconnected even when the routing program is switched over, so the neighboring router operates with the routes retained.

• Route learning and route advertisement processing sequences are fixed. When graceful restart is performed, routing information is first learned from neighboring routers, and route advertisement starts after route learning is completed. This prevents routes from disappearing from neighboring routers by advertising only some routes.

A router that performs a graceful restart is called a restart router.

The following figure shows the graceful restart operation procedure for the Switch.

Figure 8-22: Graceful restart procedure

1. When the master switch is switched or the routing program is restarted, each protocol starts a graceful restart. Each protocol is reconnected by a graceful restart to learn routes.

2. Each protocol subject to graceful restart completes route learning. However, if route learning is not completed within the route hold time (specified by the configuration command routing options graceful-restart time-limit) during graceful restart after graceful restart, route learning is interrupted and route advertisement starts.

3. After route learning is completed, each protocol that is subject to graceful restart starts route advertisement. However, if an extranet is used, route advertisement does not start immediately even if the following conditions occur: After 30 seconds, each protocol starts route advertisement.

   • Route learning for each protocol is completed.

   • Learning of routes within the graceful restart route hold time is timed out

4. Each protocol reverts to normal protocol operation after route advertisement is complete. When all protocols have finished advertising routes, graceful restart of the entire switch is completed.

Because the operating method of graceful restart varies depending on the protocol, the operating conditions also differ. Check the graceful restart operation conditions for each protocol before use. For the individual features of each protocol, see the following:

• OSPF

  12.5 Description of graceful restart

• BGP4

  13.4.11 Graceful restart

• OSPFv3

  28.3 Description of graceful restart

• BGP4+

  29.4.11 Graceful restart