R21xx-HP FlexFabric 11900 Layer 3 IP Routing Configuration Guide

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information. All the Level-2 and Level-1-2 routers must be contiguous to form the backbone of the IS-IS

routing domain. Level-2 routers can establish neighbor relationships even if they are in different

areas.

• Level-1-2 router—A router with both Level-1 and Level-2 router functions is a Level-1-2 router. It can

establish Level-1 neighbor relationships with Level-1 and Level-1-2 routers in the same area, and

establish Level-2 neighbor relationships with Level-2 and Level-1-2 routers in different areas. A Level-1

router can reach other areas only through a Level-1-2 router. The Level-1-2 router maintains two LSDBs,

a Level-1 LSDB for intra-area routing and a Level-2 LSDB for inter-area routing.

Figure 31 sho

ws one IS-IS network topology. Area 1 is the backbone that comprises a set of Level-2

routers. The other four areas are non-backbone areas connected to the backbone through Level-1-2

routers.

Figure 31 IS-IS topology 1

Figure 32 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.

Area 1

Area 3

Area 5

Area 4

Area 2

L1/L2

L1 L1

L1/L2