Designated IS

Designated IS

IS-IS elects a Designated IS (DIS, for short) for every broadcast network. Instead of each router in the broadcast network forming adjacency with each other, the network itself is considered a router- a Pseudonode. Each router, including the DIS, advertises a single link to the pseudonode. The DIS also advertises a link to all attached routers. All routers form adjacencies with all other routers on the broadcast network, including the DIS.

DIS Election Process:

Each IS-IS router interface is assigned a L1 and L2 priority. The default priority for both, L1 and L2, is 64. Non-broadcast interface (like Serial interface) is assigned a priority 0. The range, however, is 0 to 127. The priority can be changed using isis priority interface configuration command.

The L1 priority is advertised in L1 Hellos, while L2 priority is advertised in L2 Hellos. If the priority is 0, the router is ineligible to become the DIS. The router with the highest priority becomes the DIS. In case of a tie, the router with the numerically highest System ID becomes the DIS. There is a separate DIS for L1 and L2.

Pseudonode:

The DIS creates the Pseudonode. All routers on the broadcast network, including the DIS, form adjacencies with the Pseudonode. The DIS assigns the LAN ID (similar to System ID) to the broadcast network. The LAN ID is created by concatenating the System ID of the DIS with its Circuit ID (Pseudonode ID, in this case) for the attached network. All routers on the network will use the LAN ID assigned by the DIS.

Network topology: