Administrator Guide

On OSPFv3, the system supports only one process at a time for all platforms.
Prior to the Dell Networking OS version 7.8.1.0, the system supported one OSPFv2 and one OSPFv3 process
ID per system. OSPFv2 and OSPFv3 can coexist but you must configure them individually.
The 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 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:
Open Shortest Path First (OSPFv2 and OSPFv3) 637