Configuring graceful restart for dynamic routing failover
When an HA failover occurs, neighbor routers will detect that the cluster has failed and remove it from the network until the routing topology stabilizes. During the time the routers may stop sending IP packets to the cluster and communications sessions that would normally be processed by the cluster may time out or be dropped. Also the new primary unit will not receive routing updates and so will not be able to build and maintain its routing database.
You can configure graceful restart (also called nonstop forwarding (NSF)) as described in
RFC3623 (Graceful OSPF Restart) to solve the problem of dynamic routing failover. If graceful restart is enabled on neighbor routers, they will keep sending packets to the cluster following the HA failover instead of removing it from the network. The neighboring routers assume that the cluster is experiencing a graceful restart.
After the failover, the new primary unit can continue to process communication sessions using the synchronized routing data received from the failed primary unit before the failover. This gives the new primary unit time to update its routing table after the failover.
You can use the following commands to enable graceful restart or NSF on Cisco routers:
router ospf 1
log-adjacency-changes
nsf ietf helper strict-lsa-checking
If the cluster is running BGP, use the following command to enable graceful restart for BGP:
config router bgp
set graceful-restart enable
end
You can also add BGP neighbors and configure the cluster unit to notify these neighbors that it supports graceful restart.
config router bgp
config neighbor
edit <neighbor_address_Ipv4>
set capability-graceful-restart enable
end
end
If the cluster is running OSPF, use the following command to enable graceful restart for OSFP:
config router ospf
set restart-mode graceful-restart
end
To make sure the new primary unit keeps its synchronized routing data long enough to acquire new routing data, you should also increase the HA route time to live, route wait, and route hold values to 60 using the following CLI command:
config system ha
set route-ttl 60
set route-wait 60
set route-hold 60
end