H3C Technologies H3C S12500 Series Switches - Manual (Page 151)

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Figure 53 IS-IS topology 1
Figure 54 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 54 IS-IS topology 2
NOTE:
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).
Route leaking
An IS-IS routing domain is comprised of only one Level-2 area and multiple Level-1 areas. A Level-1 area
consists of a group of Level-1 routers, and is connected with a Level-2 area rather than other Level-1 areas.