Dell PowerSwitch S4820T 9.50.1 Configuration Guide for the S4820T System - Page 642

Dell Networking OS supports stub areas, totally stub (no summary) and not so stubby areas (NSSAs) and supports the following LSAs, as described earlier. • Router (type 1) • Network (type 2) • Network Summary (type 3) • AS Boundary (type 4) • LSA(type 5) • External LSA (type 7) • Link LSA, OSPFv3 only (type 8) • Opaque Link-Local (type 9) • Grace LSA, OSPFv3 only (type 11) Graceful Restart Graceful restart for OSPFv2 and OSPFv3 are supported on the S4820T platform in Helper and Restart modes. When a router goes down without a graceful restart, there is a possibility for loss of access to parts of the network due to ongoing network topology changes. Additionally, LSA flooding and reconvergence can cause substantial delays. It is, therefore, desirable that the network maintains a stable topology if it is possible for data flow to continue uninterrupted. OSPF graceful restart understands that in a modern router, the control plane and data plane functionality are separate, restarting the control plane functionality (such as the failover of the active RPM to the backup in a redundant configuration), does not necessarily have to interrupt the forwarding of data packets. This behavior is supported because the forwarding tables previously computed by an active RPM have been downloaded into the forwarding information base (FIB) on the line cards (the data plane) and are still resident. For packets that have existing FIB/CAM entries, forwarding between ingress and egress ports/VLANs, and so on, can continue uninterrupted while the control plane OSPF process comes back to full functionality and rebuilds its routing tables. To notify its helper neighbors that the restart process is beginning, when a router is attempting to restart gracefully, it originates the following link-local Grace LSAs: • An OSPFv2 router sends Type 9 LSAs. • An OSPFv3 router sends Type 11 LSAs. Type 9 and 11 LSAs include a grace period, which is the time period an OSPF router advertises to adjacent neighbor routers as the time to wait for it to return to full control plane functionality. During the grace period, neighbor OSPFv2 /v3 interfaces save the LSAs from the restarting OSPF interface. Helper neighbor routers continue to announce the restarting router as fully adjacent, as long as the network topology remains unchanged. When the restarting router completes its restart, it flushes the Type 9 and 11 LSAs, notifying its neighbors that the restart is complete. This notification happens before the grace period expires. Dell Networking routers support the following OSPF graceful restart functionality: • Restarting role in which an enabled router performs its own graceful restart. • Helper role in which the router's graceful restart function is to help a restarting neighbor router in its graceful restarts. 642 Open Shortest Path First (OSPFv2 and OSPFv3)