IS-IS Network Types

IS-IS Network Types
IS-IS recognizes only broadcast and point-to-point links. In Frame Relay,
multipoint interfaces must be fully meshed. Use point-to-point subinterfaces
to avoid this.
On a broadcast network, IS-IS routers elect a Designated Intermediate
System (DIS). The DIS is elected based on priority, with MAC address as
the tie breaker (the lowest number wins for both priority and MAC address).
Routers form adjacencies with the DIS and all other routers on the LAN.
The DIS creates a pseudonode to represent the network and sends out an
advertisement to represent the LAN. All routers advertise only an adjacency
to the pseudonode. If the DIS fails, another is elected; no backup DIS exists.
The DIS sends Hellos every 3.3 seconds; other routers send them every 10
seconds. The DIS also multicasts a CSNP every 10 seconds.
No DIS exists on a point-to-point link. When an adjacency is first formed
over the link, the routers exchange CSNPs. If one of the routers needs more
information about a specific network, it sends a PSNP requesting that. After
the initial exchange, LSPs are sent to describe link changes, and they are
acknowledged with PSNPs. Hellos are sent every 10 seconds.