3Com Switch 7750 Configuration Guide

IS-IS Overview 329

Figure 76 IS-IS topology I

Figure 77 shows another IS-IS network topology. The Level-1-2 routers connect

the Level-1 and Level-2 routers, and 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 77 IS-IS topology II

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).

Area 1

Area 3

Area 5

Area 4

Area 2

L1/L2

L1 L1

L1/L2

L1 L1

Area 1

Area 3

Area 2

L1/L2

Area 4