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Graceful Restart
Copyright © 2010, Juniper Networks, Inc.
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The router applies the specified route map to all routes currently and subsequently
installed in the routing table. If any previously redistributed routes are changed as a result
of applying the route map, the router redistributes these routes again with the changes
caused by the route map.
For details about configuring and using route maps, see JunosE IP Services Configuration
Guide.
E Series routers support IS-IS graceful restart as defined in RFC 3847—Restart Signaling
for Intermediate System to Intermediate System (IS-IS) (July 2004). Graceful restart is
also known as nonstop forwarding (NSF). When graceful restart is enabled on an IS-IS
router, it allows the router to restart with minimal routing disruption to the network.
Features
When a router running in an IS-IS domain restarts, it typically causes routers in that
domain to reset their adjacencies, thus generating unnecessary LSP flooding and
shortest-path-first (SPF) calculations throughout the domain. Enabling graceful restart
minimizes these effects by providing a mechanism by which a restarting router can do
the following:
Notify neighboring IS-IS routers that it is restarting and request help resynchronizing
its LSP database. Neighbors with active adjacencies to the restarting router can thereby
reestablish these adjacencies without having to reset them.
Determine when complete LSP database synchronization with its neighbors has
occurred.
Optimize the process of LSP database synchronization while minimizing temporary
routing disruption.
IS-IS graceful restart on E Series routers supports both restart and helper capabilities.
These capabilities mean that an E Series router can not only notify neighboring IS-IS
routers that it is restarting and request help resynchronizing its LSP database, but can
also cooperate with other restarting routers to help them with the restart process.
How Graceful Restart Works
Graceful restart is disabled on the router by default. When you enable graceful restart
by issuing the nsf ietf command, the router sends restart requests to neighboring routers
to notify them that it is restarting. The restarting router includes the restart TLV (type
211) in its hello PDUs to signal the other routers that it supports graceful restart and to
request help resynchronizing its LSP database. Including the restart TLV in hello packets
also ensures that neighboring routers will maintain their active adjacencies to the restarting
router and keep the restarting router in the network topology.
Chapter 6: Configuring IS-IS
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