Deployment Guide

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.
Fast Convergence (OSPFv2, IPv4 Only)
Fast convergence allows you to define the speeds at which LSAs are originated and accepted, and reduce OSPFv2 end-to-end
convergence time.
Dell EMC Networking OS allows you to accept and originate LSAs as soon as they are available to speed up route information
propagation.
NOTE:
The faster the convergence, the more frequent the route calculations and updates. This impacts CPU utilization and
may impact adjacency stability in larger topologies.
610 Open Shortest Path First (OSPFv2 and OSPFv3)