• 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 S4048–ON 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.
• Helper-reject role in which OSPF does not participate in the graceful restart of a neighbor.
OSPFv2 supports helper-only and restarting-only roles. By default, both helper and restarting roles are
enabled. OSPFv2 supports the helper-reject role globally on a router.
OSPFv3 supports helper-only and restarting-only roles. The helper-only role is enabled by default. To
enable the restarting role in addition to the helper-only role, configure a grace period. Reconfigure
OSPFv3 graceful restart to a restarting-only role when you enable the helper-reject role on an interface.
OSPFv3 supports the helper-reject role on a per-interface basis.
Open Shortest Path First (OSPFv2 and OSPFv3) 716
To Next Page
Loading...
Need help?
General
