HP MSR Series Configuration Manual page 128

Figure 29 IS-IS topology 1
Figure 30 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. No area is defined as the backbone in this
topology. The backbone comprises all contiguous Level-2 and Level- 1 -2 routers in different areas.
Figure 30 IS-IS topology 2
NOTE:
The IS-IS backbone does not need to be a specific area.
Both the Level- 1 and Level-2 routers use the SPF algorithm to generate the shortest path tree.
Route leaking
Level-2 and Level- 1 -2 routers form a Level-2 area. An IS-IS routing domain comprises only one Level-2 area
and multiple Level- 1 areas. A Level- 1 area must connect to the Level-2 area rather than other Level- 1 areas.
The routing information of each Level- 1 area is sent to the Level-2 area through a Level- 1 -2 router, so a
Level-2 router knows the routing information of the entire IS-IS routing domain. By default, a Level- 1 -2
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