Editing IS-IS (section)

== Broadcast segments and designated intermediate system ==
On broadcast networks IS-IS is prone to issue, similar to OSPF, when all routers on the broadcast segment need to form adjacency and exchange LSPs. Therefore, number of LSPs increase in square.

In order to overcome this issue, on each LAN segment a designated intermediate system (DIS) is elected. The router with the highest priority and System ID wins. But, if a new router shows up and has better priority or System ID, then it is elected as a new DIS.

Elected DIS router is a pseudonode, which uses resources (including System ID) of one real router. DIS describes adjacency between routers in the broadcast segment in hub-spoke manner, where DIS is the hub while other routers (including router, promoted to DIS) are spokes.

Pseudonode ID in LSPs, originated from DIS, always have Pseudonode ID field different from zero.

All routers on the LAN segment form adjacency with only DIS and exchanges LSPs with it. 

The function of DIS is to send periodic CSNPs on the LAN segment and reply to PSNPs from other routers. In case of DIS failure a new DIS will be elected in the segment. The role of DIS is not as critical as of DR in OSPF. That's why there is no backup DIS (BDIS) elected in IS-IS compared to BDR in OSPF.