3Com MSR 50 Series Configuration Manual page 880

880 C 57: IS-IS C
HAPTER ONFIGURATION
n
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 form the backbone in a routing domain. The backbone must be physically
contiguous. 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.
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 254 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.
Figure 254 IS-IS topology
Area 2
L1/L2
L1
Area 4
L1
Figure 255 shows another network topology running the IS-IS protocol. The
Level-1-2 routers connect the Level-1 and Level-2 routers, and also form the IS-IS
L2
Area 1
L2
L1/L2
L1/L2
L1
Area 3
L1/L2
L2
L2
Area 5
L1
L1
L1