H3C Technologies H3C S7500E Series Switches - Manual (Page 150)

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Figure 5-2 IS-IS topology
Figure 5-3 shows another IS-IS topology. The Level-1-2 routers connect to 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 comprises all contiguous Level-2 and Level-1-2 routers
which can reside in different areas.
Figure 5-3 IS-IS topology
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).