IS-IS network types

IS-IS supports only broadcast networks (for example, Ethernet and Token Ring).

IS-IS routers on a broadcast network must elect a DIS.

The Level-1 and Level-2 DISs are elected separately. You can assign different priorities to a router for different level DIS elections. The higher the router priority, the more likely the router becomes the DIS. If multiple routers with the same highest DIS priority exist, the one with the highest Subnetwork Point of Attachment (SNPA) address will be elected. On a broadcast network, the SNPA address is the MAC address. A router can be the DIS for different levels.

IS-IS DIS election differs from OSPF DIS election in the following ways:

• A router with priority 0 can also participate in the DIS election.

• When a router with a higher priority is added to the network, an LSP flooding process is performed to elect the router as the new DIS.

As shown in Figure 38, the same level routers on a network, including non-DIS routers, establish adjacency with each other.

The DIS creates and updates pseudonodes, and generates LSPs for the pseudonodes, to describe all routers on the network.

A pseudonode represents a virtual node on the broadcast network. It is not a real router. In IS-IS, it is identified by the system ID of the DIS and a 1-byte Circuit ID (a non-zero value).

Using pseudonodes simplifies network topology and can reduce the amount of resources consumed by SPF.

	NOTE: On an IS-IS broadcast network, all routers establish adjacency relationships, but they synchronize their LSDBs through the DIS.