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IS-IS Overview Configuring IS-IS
page 3-12 OmniSwitch 6800/6850/9000 Advanced Routing Configuration Guide December 2007
Graceful Restart on Stacks with Redundant Switches
OmniSwitch 6850 stacks with two or more switches support redundancy, where if the primary switch fails
or goes offline, the secondary switch is instantly notified. The secondary switch automatically assumes the
primary role. This transition from secondary to primary is known as takeover.
When the router is in the graceful restart mode, it informs its neighbors of the restart. The IS-IS Hello
(IIH) messages are modified to signal a graceful restart request. The neighbors respond by sending back
their own IIHs with an acknowledgement of the restart, along with a "Remaining Time" value to indicate
how long they will wait for a restart. The neighbors also continue to send out LSPs with the restarting
router still listed as an adjacency, thus avoiding SPF calculations and enabling traffic to flow to the router
from neighbors.
The restarting router continues to forward LSPs using its pre-restart forwarding tables. When graceful
restart is enabled, the router can either be a helper or a restarting router, or both. In the current release,
only the helper mode is supported. If a helper is enabled on a neighbor, it begins the Link State Database
synchronization process. They send their Complete Sequence Number PDUs (CSNPs) to the restarting
router. The restarting router can then determine the LSPs it needs and request them. After it receives all
requested LSPs, the database is synchronized.
Note. When graceful restart is enabled on the router, the helper mode is automatically enabled by default.
When the graceful restart timer expires, the restarting router runs the SPF calculation to re-compute IS-IS
routes. Only then does it flood LSPs to neighbors and comes back to normal protocol behavior.
In the network illustration below, a helper router, Router Y, monitors the network for topology changes.
As long as there are none, it continues to advertise its LSPs as if the restarting router, Router X, had
remained in continuous IS-IS operation (i.e., Router Y’s LSPs continue to list an adjacency to Router X
over network segment S, regardless of the adjacency’s current synchronization state).
IS-IS Graceful Restart Helper and Restarting Router
Restarting Router X
Network Segment S
Helper Router Y
Router A
Router B
Router C