Dell PowerConnect 6248 Configuration Guide - Page 175
Routed Access Scenario, NSF and Routed Access
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Routed Access Scenario Figure 9-5 shows a stack of three units serving as an access router for a set of hosts. Two LAGs connect the stack to two aggregation routers. Each LAG is a member of a VLAN routing interface. The stack has OSPF and PIM adjacencies with each of the aggregation routers. The top unit in the stack is the management unit. Figure 9-5. NSF and Routed Access ` LAG1 ` Access Router LAG2 Aggregation Routers If the management unit fails, its link to the aggregation router is removed from the LAG. When the control plane restarts, both routing interfaces come back up by virtue of the LAGs coming up. OSPF sends grace LSAs to inform its OSPF neighbors (the aggregation routers) that it is going through a graceful restart. NOTE: The graceful restart feature for OSPF is disabled by default. To enable OSPF to perform a graceful restart for all planned and unplanned warm restart events, use the nsf command in Router OSPF Config mode: console#configure console(config)#router ospf console(config-router)#nsf The grace LSAs reach the neighbors before they drop their adjacencies with the access router. PIM starts sending hello messages to its neighbors on the aggregation routers using a new generation ID to prompt the neighbors to quickly resend multicast routing information. PIM neighbors recognize the new generation ID and immediately relay the group state back to the restarting router. IGMP sends queries to relearn the hosts' interest in multicast groups. IGMP tells PIM the group membership, and PIM sends JOIN messages upstream. The control plane updates the driver with checkpointed unicast routes. The forwarding plane reconciles L3 hardware tables. The OSPF graceful restart finishes, and the control plane deletes any stale unicast routes not relearned at this point. The forwarding plane reconciles L3 multicast hardware tables. Throughout the process, the hosts continue to receive their multicast streams, possibly with a short interruption as the top aggregation router learns that one it its LAG members is down. The hosts see no more than a 50 ms interruption in unicast connectivity. Utility 175