HP 4800G Series Configuration Manual page 520

IS-IS Graceful Restart
For detailed GR information, refer to GR Overview in the High Availability Volume.
After an IS-IS GR Restarter restarts IS-IS, it needs to complete the following two tasks to synchronize
the LSDB with its neighbors.
To obtain effective IS-IS neighbor information without changing adjacencies.
To obtain the LSDB contents.
After the restart, the GR Restarter will send an OSPF GR signal to its neighbors to keep the adjacencies.
After receiving the responses from neighbors, the GR Restarter can restore the neighbor table.
After reestablishing neighborships, the GR Restarter will synchronize the LSDB and exchange routing
information with all adjacent GR capable neighbors. After that, the GR Restarter will update its own
routing table and forwarding table based on the new routing information and remove the stale routes. In
this way, the IS-IS routing convergence is complete.
Management tag
Management tag simplifies routing information management by carrying the management information
of the IP address prefixes (to control route redistribution from other routing protocols) and BGP
community and extended community attributes.
LSP fragment extension
IS-IS advertises link state information by flooding LSPs. One LSP carries a limited amount of link state
information; therefore, IS-IS fragments LSPs. Each LSP fragment is uniquely identified by a
combination of the System ID, Pseudonode ID (0 for a common LSP or a non-zero value for a
Pseudonode LSP), and LSP Number (LSP fragment number) of the node or pseudo node that
generated the LSP. The one-byte LSP Number field, allowing a maximum of only 256 fragments to be
generated by an IS-IS router, limits the amount of link information that the IS-IS router can advertise.
The LSP fragment extension feature allows an IS-IS router to generate more LSP fragments. Up to 50
additional virtual systems can be configured on the router, and each virtual system is capable of
generating 256 LSP fragments to enable the IS-IS router to generate up to 13056 LSP fragments.
1) Terms
Originating System
It is the router actually running IS-IS. After LSP fragment extension is enabled, additional virtual
systems can be configured for the router. Originating system is the actual IS-IS process that originally
runs.
System ID: System ID of the originating system.
Additional System ID
Additional virtual system IDs are configured for the IS-IS router after LSP fragment extension is enabled.
Each additional system ID can generate 256 LSP fragments. Both the additional system ID and the
system ID must be unique in the entire routing domain.
Virtual System
1-13