KB.15.15

The configuration is shown graphically in Figure 269 (page 483).

Figure 269 Example of nonstop routing configuration

Nonstop forwarding with RIP

On a Nonstop RIP router, the traffic does not get re-routed when the MM fails over. A request

packet is sent on failover that asks for the router’s peers to send routing updates to the requesting

router. There is no loss of routed traffic.

Nonstop forwarding with OSPFv2 and OSPFv3

On a Nonstop OSPFv2 router, failover of a MM does not result in the OSPF v2 router being

removed from the OSPFv2 domain. A restart request is sent by the Nonstop OSPFv2 router to the

neighboring OSPFv2 routers, after which the graceful restart process begins. This behavior applies

to OSPFv3 as well.

A graceful restart allows an OSPF routing switch to stay on the forwarding path while being

restarted. The routing switch sends “grace LSAs” that notify its neighbors that it intends to perform

a graceful restart. During the configurable grace period, the restarting switch’s neighbors continue

to announce the routing switch in their LSAs as long as the network topology remains unchanged.

The neighbors run in “helper mode” while the routing switch restarts.

Graceful restart will fail under these conditions:

• There is a topology change during the graceful restart period. The helper switches exit helper

mode and adjacencies are lost until the restarting switch rebuilds the adjacencies.

• The neighbor switches do not support helper mode.

For more information on OSPFv2 and OSPFv3 graceful restart, see RFC 3623 and RFC 5187.

Enabling nonstop forwarding for OSPFv2

The routing switch must be in ospf context when enabling Nonstop forwarding for OSPFv2. To

enable Nonstop forwarding, enter this command.

Nonstop forwarding with RIP 483