328 CHAPTER 29: IS-IS CONFIGURATION
Level-1 and Level-2
1 Level-1 router
The Level-1 router only establishes the neighbor relationship 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 out of the area to
the nearest Level-1-2 router.
2 Level-2 router
The Level-2 router establishes the 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 in a routing domain. Only
Level-2 routers can directly communicate with routers outside the routing domain.
3 Level-1-2 router
A router with both Level-1 and Level-2 router functions is called a Level-1-2 router.
It can establish the Level-1 neighbor relationship with the Level-1 and Level-1-2
routers in the same area, or establish Level-2 neighbor relationship with the
Level-2 and Level-1-2 routers in different areas. A Level-1 router must be
connected to other areas via 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.
n
■ The Level-1 routers in different areas can not establish the neighbor
relationship.
■ The neighbor relationship establishment of Level-2 routers has nothing to do
with area.
Figure 100 shows a network topology running the IS-IS protocol. Area 1 is a set of
Level-2 routers, called backbone network. The other four areas are non-backbone
networks connected to the backbone through Level-1-2 routers.
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