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Utility 175
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
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.
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LAG1
LAG2
Access Router
Aggregation Routers