3Com S7902E, Configuration Manual

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1-5
Figure 1-3 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).
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.
The routing information of a 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 of other Level-1 areas and the Level-2 area with the Level-1 area by default.
Since a Level-1 router simply sends packets destined for other areas to the nearest Level-1-2 router,
this may cause that the best paths cannot be selected.
To solve this problem, route leaking was introduced. A Level-2 router can advertise Level-2 routing
information to a specified Level-1 area. By having the routing information of other areas, a Level-1
router in the area can make a better routing decision for a packet to another area.
IS-IS Network Type
Network type
IS-IS supports two network types:
z
Broadcast network, such as Ethernet, Token-Ring.
z
Point-to-point network, such as PPP, HDLC.