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Dell Configuration Guide for the S4048–ON System 9.14.2.6

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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 EMC 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.

    Configuring helper-reject role on an OSPFv2 router or OSPFv3 interface enables the restarting-only role globally on the router or locally on the interface. In a helper-reject role, OSPF does not participate in the graceful restart of an adjacent OSPFv2/v3 router.

    If multiple OSPF interfaces provide communication between two routers, after you configure helper-reject on one interface, all other interfaces between the two routers behave as if they are in the help-reject role.

  • OSPFv2 and OSPFv3 support planned-only and/or unplanned-only restarts. The default is support for both planned and unplanned restarts.

    A planned restart occurs when you enter the redundancy force-failover rpm command to force the primary RPM to switch to the backup RPM. During a planned restart, OSPF sends out a Grace LSA before the system switches over to the backup RPM.

    An unplanned restart occurs when an unplanned event causes the active RPM to switch to the backup RPM, such as when an active process crashes, the active RPM is removed, or a power failure happens. During an unplanned restart, OSPF sends out a Grace LSA when the backup RPM comes online.

To display the configuration values for OSPF graceful restart, enter the show run ospf command for OSPFv2 and the show run ospf and show ipv6 ospf database database-summary commands for OSPFv3.

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