Understanding IS-IS Designated Routers

A router advertises its priority to become a designated router in its hello packets. On all multiaccess networks (physical networks that support the attachment of more than two routers, such as Ethernet networks), IS-IS uses the advertised priorities to elect a designated router for the network. This router is responsible for sending network link-state advertisements, which describe all the routers attached to the network. These advertisements are flooded throughout a single area. The priority value is meaningful only on a multiaccess network. It has no meaning on a point-to-point interface.

A router’s priority for becoming the designated router is indicated by an arbitrary number from 0 through 127, which you configure on the IS-IS interface. The router with the highest priority becomes the designated router for the area (Level 1, Level 2, or both), also configured on the IS-IS interface. If routers in the network have the same priority, then the router with the highest MAC address is elected as the designated router. By default, routers have a priority value of 64.

In IS-IS protocol, a pseudo LSP (Link State PDU) is used to represent a pseudo node, which is a logical node that is not a real physical device. The designated router becomes a pseudonode and advertises pseudonode LSPs. IS-IS modifies the deployment as a hub-and-spoke topology with all nodes in the same broadcast domain having a point-to-point connection with a pseudonode. This helps in avoiding a full-mesh of point-to-point connections between all nodes in the same LAN segment and simplifies the network topology.

Each LSP is identified by its <name>.<pseudonodeid>-<fragment-id>. You can distinguish between a regular LSP and a pseudo LSP by checking the <pseudonodeid> value. A regular LSP has a <pseudonodeid> value of zero, whereas a pseudo LSP has a non-zero value in its LSP packet.