3Com Switch 4800G 24-Port Configuration Manual page 329

Figure 100 IS-IS topology
Area 2
L1/L2
L1
Area 4
L1
Figure 101 shows another network topology running the IS-IS protocol. The
Level-1-2 routers connect the Level-1 and Level-2 routers, and also form the IS-IS
backbone together with the Level-2 routers. There is no area defined as the
backbone in this topology. The backbone is composed of all contiguous Level-2
and Level-1-2 routers which can reside in different areas.
Figure 101 IS-IS topology
L1
Area 2
L1
n
The IS-IS backbone does not need to be a specific Area.
Both the IS-IS Level-1 and Level-2 routers use the SPF algorithm to generate the
Shortest Path Tree (SPT).
Interface routing hierarchy type
You can configure the routing type for each interface. For a Level-1-2 router, one
interface may establish Level-1 adjacency with a router, and another one may
establish Level-2 adjacency with another router. You can limit the adjacency type
L2
L2
L1/L2
L1
Area 1
L2
L1/L2
L1/L2
Area 3
L2
IS-IS Overview
Area 3
L1/L2
L2
Area 1
L2
Area 5
L1/L2
L1
Area 4
L1
329
L1
L1