H3C Technologies H3C S7500E Series Switches - Manual (Page 149)

5-4
Level-1 and Level-2
1) Level-1 router
A Level-1 router establishes neighbor relationships with Level-1 and Level-1-2 routers in the same
area. The LSDB maintained by the Level-1 router contains the local area routing information. It directs
the packets destined for an outside area to the nearest Level-1-2 router.
2) Level-2 router
A Level-2 router establishes neighbor relationships with the Level-2 and Level-1-2 routers in the same
or in different areas. It maintains a Level-2 LSDB which contains inter-area routing information. All the
Level-2 and Level-1-2 routers must be contiguous to form the backbone of a routing domain.
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.
z The Level-1 routers in different areas can not establish neighbor relationships.
z The neighbor relationship establishment of Level-2 routers has nothing to do with area.
Figure 5-2 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.