To control the amount of IS-IS protocol traffic sent within the local network, IS-IS networks are divided into areas, just as OSPF networks have areas. Each IS-IS area consists of a set of networks and routers that are administratively grouped together.

All the routers within an area exchange their network topology information in IS-IS link-state protocol data units (LSPs), and this smaller group of routers run the shortest path first (SPF) calculation to keep their link-state databases identical.

Routers within an area share the information in their link-state databases with each other by exchanging LSPs. This process ensures that all the link-state databases in the area are identical; hence all routers within an area have the same view of the area’s network topology. Routers within an area can send summaries of their routes to other areas in the IS-IS network.

IS-IS areas contain two types of routers:

  • Level 1 systems: These routers route traffic within an IS-IS area. When they receive traffic destined for somewhere outside the area, they send the packet toward a Level 2 system.

  • Level 2 systems: These routers route traffic between two IS-IS areas. They also route traffic to other ASs.

A single IS-IS router can be both a Level 1 and a Level 2 system, which is similar to the OSPF area border router. These routers maintain two link-state databases, one for the Level 1 area and a second one for the Level 2 area.