340 CHAPTER 32: INTEGRATED IS-IS CONFIGURATION
Two-level Structure of
IS-IS Routing Protocol
Two-level structure of IS-IS routing protocol
Two-level structure of IS-IS routing protocol is adopted in a route area to support
large scale route network. A large route area can be divided into one or multiple
areas. A Level-1 router manages the intra-area routes. A Level-2 router manages
the inter-area routes.
Level-1 and Level-2
■ Level-1 router
The Level-1 router is responsible for intra-area route. The Level-1 router and the
Level-1 router or Level-1-2 router in the same area are neighbors. The Level-1
router maintains a Level-1 LSDB. This LSDB contains intra-area routing
information. The packets sent to other areas are forwarded to the closest Level-2
router.
■ Level-2 router
The Level-2 router is responsible for inter-area route. The Level-2 router and
Level-2 routers or Level-1-2 routers in other areas are neighbors. The Level-2
router maintains a Level-2 LSDB. This LSDB contains inter-area routing information.
The backbone (which is made up of all Level-2 routers) of a route area is
responsible for inter-area communications. The Level-2 routers in the route area
must be continuous to ensure the backbone continuity.
■ Level-1-2 router
A Level-1-2 router is both a Level-1 router and a Level-2 router. At least one
Level-1-2 router in each area connects the area to the backbone network. A
Level-1-2 router maintains two LSDBs: the Level-1 LSDB for intra- area route and
Level-2 LSDB for inter-area route.
Figure 87 illustrates a network running IS-IS routing protocol and composed of
Routing Domain 1 and Routing Domain 2. Routing Domain 1 includes two areas,
Area 1 and Area 2, and Routing Domain 2 only has Area 3. In Routing Domain 1,
the three ISs connected by bold lines compose the area backbone. They are all
Level-2 routers. The other 4 ISs not connected by bold line are Level-1-2 routers.
To Next Page
Loading...
