3com S7906E Manuel De Montage

 

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). 

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 supports two network types: 

z 

Broadcast network, such as Ethernet, Token-Ring. 

z 

Point-to-point network, such as PPP, HDLC.