In IS-IS routing protocol, a single router is chosen to be the designated intermediate system (DIS). The election of the DIS is determined automatically and dynamically on the LAN depending on highest interface priority and the subnetwork point of attachment (SNPA). The FortiGate is typically the DIS, and each router in its LAN is an intermediate system (IS).

Unlike OSPF, which elects a designated router (DR) and backup designated router (BDR), the DIS has no backup and determines the election of a new DIS whenever a router is added to the LAN or whenever the current DIS drops. A backup DIS is irrelevant since all of the routers on an IS-IS system are synchronized, and the short Hello interval used by the DIS quickly detects failures and the subsequent replacement of the DIS.

Synchronization of all the nodes in an IS-IS area could prove troublesome when updating the network infrastructure, and would demand ever-increasing resources each time a new router is added (at an exponential scale). For this purpose the DIS creates a pseudonode, which is essentially a virtual, logical node representing the LAN. The pseudonode requests adjacency status from all the routers in a multi-access network by sending IS-IS Hello (IIH) PDUs to Level 1 and Level 2 routers (where Level 1 routers share the same address as the DIS and Level 2 routers do not). Using a pseudonode to alter the representation of the LAN in the link-state database (LSD) greatly reduces the amount of adjacencies that area routers have to report. In essence, a pseudonode collapses a LAN topology, which allows a more linear scale to link-state advertising.

In order to maintain the database synchronization, the DIS periodically sends complete sequence number packets (CSNPs) to all participating routers.