3Com Switch 8800 Configuration Guide
Chapter 20 Integrated IS-IS Configuration
20-2
II. Link types IS-IS routing protocol is applied to
IS-IS routing protocol can run on point to point Links, such as PPP, HDLC and others.
IS-IS routing protocol can also run on broadcast links, such as Ethernet, Token-Ring
and others. For a Non-Broadcast Multi-Access (NBMA) network such as ATM, you
need to configure sub-interfaces and configure sub-interface type as P2P or broadcast
network. IS-IS routing protocol cannot run on point to MultiPoint links.
20.1.2 Two-level Structure of IS-IS Routing Protocol
I. 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.
II. Level-1 and Level-2
z
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.
z
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.
z
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 20-1 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 routers.
