Administrator Guide

Layer 3 Routing Commands 1699
nsf helper strict-lsa-checking
Use the nsf-helper strict-lsa-checking command to require that an OSPF
helpful neighbor exit helper mode whenever a topology change occurs. Use
the “no” form of this command to allow OSPF to continue as a helpful
neighbor in spite of topology changes.
Syntax
nsf [ietf] helper strict-lsa-checking
no nsf [ietf] helper strict-lsa-checking
• ietf —This keyword is used to distinguish the IETF standard
implementation of graceful restart from other implementations. Since the
IETF implementation is the only one supported, this keyword is optional.
Default Configuration
A helpful neighbor exits helper mode when a topology change occurs.
Command Mode
Router OSPF Configuration mode
User Guidelines
The restarting router is unable to react to topology changes. In particular, the
restarting router will not immediately update its forwarding table; therefore, a
topology change may introduce forwarding loops or black holes that persist
until the graceful restart completes. By exiting the graceful restart on a
topology change, a router tries to eliminate the loops or black holes as quickly
as possible by routing around the restarting router.
A helpful neighbor considers a link down with the restarting router to be a
topology change, regardless of the strict LSA checking configuration.
nsf restart-interval
Use the nsf restart-interval command to configure the length of the grace
period on the restarting router. Use the “no” form of this command to revert
the grace period to its default.