3Com MSR 50 Series Configuration Manual page 881

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 255 IS-IS topology
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).
Interface routing hierarchy type
You can configure the routing type for each interface. For a Level-1-2 router, one
interface may establish Level-1 adjacency with a router, and another one may
establish Level-2 adjacency with another router. You can limit the adjacency type
by configuring the routing hierarchy on the interface. For example, the level-1
interface can only establish Level-1 adjacency, while the level-2 interface can only
establish Level-2 adjacency.
By having this function, you can prevent the Level-1 hello packets from
propagating to the Level-2 backbone through the Lever-1-2 router. This can result
in bandwidth saving.
Route leaking
An IS-IS routing domain is comprised of only one Level-2 area and multiple Level-1
areas. A Level-1 area is connected with the Level-2 area rather than other Level-1
areas.
The routing information of the Level-1 area is sent to the Level-2 area through the
Level-1-2 router. Therefore, the Level-2 router knows the routing information of
the entire IS-IS routing domain but does not share the information with the
Level-1 area by default.
Since the Level-1 router simply sends the routing information for destinations
outside the area to the nearest Level-1-2 router, this may cause a problem that the
best path cannot be selected.
To solve this problem, route leaking was introduced. The Level-2 router can
advertise the Level-2 routing information to a specified Level-1 area. By having the
Area 1
L2
L1/L2
L1/L2
L2
IS-IS Overview
Area 4
L1
Area 3
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