Supported IS-IS features

Multiple instances and processes

IS-IS supports multiple instances and processes. Multiple processes allow an IS-IS process to work in concert with a group of interfaces. A router can run multiple IS-IS processes, and each process corresponds to a unique group of interfaces.

For routers supporting VPN, each IS-IS process is associated with a VPN instance. The VPN instance is also associated with interfaces of the process.

IS-IS Graceful Restart

Graceful Restart (GR) ensures the continuity of packet forwarding when a routing protocol restarts or an active/standby switchover occurs:

After an IS-IS GR Restarter restarts, it must complete the following tasks to synchronize the LSDB with its neighbors:

The GR Restarter sends an OSPF GR signal to GR Helpers so that the GR Helpers keep their adjacencies with the GR Restarter, and restores the neighbor table after receiving responses from neighbors. The GR Restarter then synchronizes the LSDB with all GR capable neighbors, calculates routes, updates its routing table and forwarding table, and removes stale routes. The IS-IS routing convergence is then complete.


Nonstop routing (NSR) is a new feature that overcomes the application limit of GR. It backs up IS-IS link state information from the master device to the slave device. When a master/slave switchover occurs, NSR can complete link state recovery and route re-generation without requiring the cooperation of other devices.

IManagement 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. Because one LSP carries a limited amount of link state information, 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 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.

Dynamic host name mapping mechanism

The dynamic host name mapping mechanism provides the mappings between the host names and the system IDs for the IS-IS routers. The dynamic host name information is announced in the dynamic host name CLV of an LSP.

This mechanism also provides the mapping between a host name and the DIS of a broadcast network, which is announced in the dynamic host name TLV of a pseudonode LSP.

A host name is easier to remember than a system ID. After enabling this feature on the router, you can see the host names instead of system IDs using the display command.


Bidirectional forwarding detection (BFD) provides a single mechanism to quickly detect any link failures between IS-IS neighbors to reduce network convergence time. For more information about BFD, see High Availability Configuration Guide.