3com 8807 User Guide

C

HAPTER

 32: I

NTEGRATED

 IS-IS C

ONFIGURATION

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.

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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.

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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.

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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.