IS-IS area
IS-IS has a 2-level hierarchy to support large-scale networks. A large-scale routing domain is divided into multiple areas. Typically, a Level-1 router is deployed within an area, a Level-2 router is deployed between areas, and a Level-1-2 router is deployed between Level-1 and Level-2 routers.
Level-1 and Level-2
Level-1 router—A Level-1 router establishes neighbor relationships with Level-1 and Level-1-2 routers in the same area. It maintains an LSDB comprising intra-area routing information. A Level-1 router forwards packets destined for external areas to the nearest Level-1-2 router. Level-1 routers in different areas cannot establish neighbor relationships.
Level-2 router—A Level-2 router establishes neighbor relationships with Level-2 and Level-1-2 routers in the same area or in different areas. It maintains a Level-2 LSDB containing inter-area routing information. All the Level-2 and Level-1-2 routers must be contiguous to form the backbone of the IS-IS routing domain. Level-2 routers can establish neighbor relationships even if they are in different areas.
Level-1-2 router—A router with both Level-1 and Level-2 router functions is a Level-1-2 router. It can establish Level-1 neighbor relationships with Level-1 and Level-1-2 routers in the same area, and establish Level-2 neighbor relationships with Level-2 and Level-1-2 routers in different areas. A Level-1 router can reach other areas only through a Level-1-2 router. The Level-1-2 router maintains two LSDBs, a Level-1 LSDB for intra-area routing and a Level-2 LSDB for inter-area routing.
Figure 33 shows one IS-IS network topology. Area 1 is the backbone that comprises a set of Level-2 routers. The other four areas are non-backbone areas connected to the backbone through Level-1-2 routers.
Figure 33: IS-IS topology 1
Figure 34 shows another IS-IS topology. The Level-1-2 routers connect to the Level-1 and Level-2 routers, and form the IS-IS backbone together with the Level-2 routers. No area is defined as the backbone in this topology. The backbone comprises all contiguous Level-2 and Level-1-2 routers in different areas. The IS-IS backbone does not need to be a specific area.
Figure 34: IS-IS topology 2
Both the Level-1 and Level-2 routers use the SPF algorithm to generate the shortest path tree.
Route leaking
Level-2 and Level-1-2 routers form a Level-2 area. An IS-IS routing domain comprises only one Level-2 area and multiple Level-1 areas. A Level-1 area must connect to the Level-2 area rather than other Level-1 area.
The routing information of each Level-1 area is sent to the Level-2 area through a Level-1-2 router, so a Level-2 router knows the routing information of the entire IS-IS routing domain. By default, a Level-2 router does not advertise the routing information of other Level-1 areas and the Level-2 area to a Level-1 area, so a Level-1 router simply sends packets destined for other areas to the nearest Level-1-2 router. The path passing through the Level-1-2 router might not be the best. To solve this problem, IS-IS provides the route leaking feature.
Route leaking enables a Level-1-2 router to advertise the routes of other Level-1 areas and the Level-2 area to the connected Level-1 area so that the Level-1 routers can select the optimal routes for packets.