3Com 7757 Configuration Manual page 329

Figure 76 IS-IS topology I
Area 2
L1/L2
L1
Area 4
L1
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
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).
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