Graceful Restart and High Availability (S-, 7100-Series)

S- and 7100-Series platforms support single router high availability failover using the following components:

• OSPF graceful restart
• Non-stop router frame forwarding on each module
• Single router configuration
• Router protocol process failover to another module
• Link Aggregate Group (LAG) connectivity to neighboring routers

In a stable network, the route and rule information is fairly constant. If the router protocol process was to suddenly fail, forwarding information current at the time of the failure in all probability is usable for the short time after the failure until recovery occurs. During this recovery period, existing connections (that were not directly using the failed module) remain in effect. New connections continue to be installed using the last known “good” forwarding information. The router protocol process that failed is dynamically restarted. You do not configure where the router process is running. The router forwarding process remains active on every module. The protocol process exchanges protocol and maintains state that it distributes to the other modules and does not have to run on any specific module. One exception to this rule is that the module must have 256M of memory to be router protocol process eligible.

Upon failure of a module running the router protocol process, the protocol process is started on a recovery module. One of the first messages it sends to its OSPF neighbors is a grace LSA. High availability failover will successfully occur if the following is true:

• The router is enabled for graceful restart
• The neighbors are enabled to participate as graceful restart helpers
• The OSPF dead interval is configured for a sufficient period such that the grace LSA is received by its neighbors before the configured OSPF dead interval expires
• And each neighbor is a member of a LAG common to the failed router, allowing the neighbor to remain up
  

  Physical and Logical Single Router HA Failover Configuration

  

Physical and Logical Single Router HA Failover Configuration depicts the physical and logical configurations of the single router high availability failover mechanism. The neighbor to router lines display direct neighbor connections to the router enabled for OSPF graceful restart and members of LAGs common to the failing router. See Link Aggregation Control Protocol (LACP) Configuration for LAG configuration details. The server 100.1.1.3 and 100.1.1.5 to router lines display VLAN connections common to both the failing and recovery routers. Helper mode on each neighbor is enabled by default. Enable graceful restart on the router using the graceful-restart enable command.

System(rw-config)->router ospf 1
System(rw-config-ospf-1)->graceful-restart enable
System(rw-config-ospf-1)->