OSI IS-IS routing uses two-level hierarchical routing. A routing domain can be partitioned into areas. Level 1 routers know the topology in their area, including all routers and end systems in their area but do not know the identity of routers or destinations outside of their area. Level 1 routers forward traffic with destinations outside of their area to a level 2 router in their area.
Level 2 routers know the level 2 topology, and know which addresses are reachable by each level 2 router. Level 2 routers do not need to know the topology within any level 1 area, except to the extent that a level 2 router can also be a level 1 router within a single area. By default, only level 2 routers can exchange PDUs or routing information directly with external routers located outside the routing domain.
The two types of routers in IS-IS are described below.
Level 1 intermediate systems
Routing is performed based on the area ID portion of the ISO address called the Network Entity Title (NET). Level 1 systems route within an area. They recognize, based on the destination address, whether the destination is within the area. If so, they route toward the destination. If not, they route to the nearest level 2 router.
Level 2 intermediate systems
Routing is performed based on the area address. They route toward other areas, regardless of other area’s internal structure. A level 2 intermediate system can also be configured as a level 1 intermediate system in the same area.
The level 1 router’s area address portion is manually configured (see ISO network addressing). A level 1 router does not become a neighbor with a node that does not have a common area address. However, if a level 1 router has area addresses A, B, and C, and a neighbor has area addresses B and D, then the level 1 router accepts the other node as a neighbor, as address B is common to both routers. Level 2 adjacencies are formed with other level 2 nodes whose area addresses do not overlap. If the area addresses do not overlap, the link is considered by both routers to be level 2 only and only level 2 LSPDUs flow on the link.
Within an area, level 1 routers exchange LSPs which identify the IP addresses reachable by each router. Specifically, zero or more IP address, subnet mask, and metric combinations can be included in each LSP. Each level 1 router is manually configured with the IP address, subnet mask, and metric combinations, which are reachable on each interface. A level 1 router routes as follows:
if a specified destination address matches an IP address, subnet mask, or metric reachable within the area; the PDU is routed via level 1 routing
if a specified destination address does not match any IP address, subnet mask, or metric combinations listed as reachable within the area; the PDU is routed toward the nearest level 2 router
Level 2 routers include in their LSPs, a complete list of IP address, subnet mask, and metrics specifying all the IP addresses which reachable in their area. This information can be obtained from a combination of the level 1 LSPs (by level 1 routers in the same area). Level 2 routers can also report external reachability information, corresponding to addresses reachable by routers in other routing domains or autonomous systems.