R21xx-HP FlexFabric 11900 Layer 3 IP Routing Configuration Guide

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Figure 32 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 area.

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-2 router

does not advertise the routing information of other Level-1 areas and the Level-2 area to a Level-1 area, so

a Level-1 router simply sends packets destined for other areas to the nearest Level-1-2 router. The path

passing through the Level-1-2 router may not be the best. To solve this problem, IS-IS provides the route

leaking feature.

Route leaking enables a Level-1-2 router to advertise the routes of other Level-1 areas and the Level-2 area

to the connected Level-1 area so that the Level-1 routers can select the optimal routes for packets.

IS-IS network types

Network types

IS-IS supports the broadcast network (for example, Ethernet and Token Ring) and the point-to-point

network (for example, PPP and HDLC).

For an NBMA interface, such as an ATM interface, you must configure point-to-point or broadcast

subinterfaces. IS-IS cannot run on P2MP links.

Area 1

Area 3

Area 5

Area 4

Area 2

L1/L2

L1 L1

L1/L2