HP VPN Firewall Appliances Appendix Protocol Reference

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Figure 11 IS-IS topology 1
Figure 12 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. The IS-IS
backbone does not need to be a specific area.
Figure 12 IS-IS topology 2
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
router does not advertise the routing information of other Level-1 areas and the Level-2 area to a Level-1