ISO specifications call routers intermediate systems. Thus, IS-IS is a router-to-router protocol, allowing routers to communicate with other routers.
IS-IS routing takes place at two levels within an AS: Level 1 (L1) and Level 2 (L2). L1 routing occurs within an IS-IS area and is responsible for routing inside an area. All devices in an L1 routing area have the same area address. Routing within an area is accomplished by looking at the locally significant address portion, known as the system ID, and choosing the lowest-cost path.
L2 routing occurs between IS-IS areas. L2 routers learn the locations of L1 routing areas and build an interarea routing table. L2 routers use the destination area address to route traffic using the lowest-cost path. Therefore, IS-IS supports two routing levels:
■ L1 builds a common topology of system IDs in the local area and routes traffic within the area using the lowest-cost path.
■ L2 exchanges prefix information (area addresses) between areas and routes traffic to an area using the lowest-cost path.
To support the two routing levels, IS-IS defines three types of routers:
■ L1 routers use link-state packets (LSP) to learn about paths within the areas to which they connect (intra-area).
■ L2 routers use LSPs to learn about paths among areas (interarea).
■ Level 1/Level 2 (L1/L2) routers learn about paths both within and between areas. L1/L2 routers are equivalent to ABRs in OSPF.
The three types of IS-IS routers are shown in Figure 7-11. Figure 7-11 Three Types of IS-IS Routers
The path of connected L2 and L1/L2 routers is called the backbone. All areas and the backbone must be contiguous.
IS-IS area boundaries fall on the links, not within the routers. Each IS-IS router belongs to exactly one area. Neighboring routers learn that they are in the same or different areas and negotiate appropriate adjacencies—L1, L2, or both.
Changing Level 1 routers into Level 1/Level 2 or Level 2 routers can easily expand the Integrated IS-IS backbone. In comparison, in OSPF, entire areas must be renumbered to achieve this.
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