3Com S7906E Configuration Manual page 715

3) 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 the Level-1 and Level-1-2 routers in the same area, or establish Level-2
neighbor relationships with the Level-2 and Level-1-2 routers in different areas. A Level-1 router must
be connected to other areas through a Level-1-2 router. The Level-1-2 router maintains two LSDBs,
where the Level-1 LSDB is for routing within the area, and the Level-2 LSDB is for routing between
areas.
The Level-1 routers in different areas can not establish neighbor relationships.
The neighbor relationship establishment of Level-2 routers has nothing to do with area.
Figure 1-2 shows an IS-IS network topology. Area 1 comprises a set of Level-2 routers and is the
backbone. The other four areas are non-backbone areas connected to the backbone through Level-1-2
routers.
Figure 1-2 IS-IS topology
Figure 1-3 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. There is no area defined as the
backbone in this topology. The backbone comprises all contiguous Level-2 and Level-1-2 routers which
can reside in different areas.
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