H3C S7500E Series Ethernet Switches Operation Manual

Operation Manual – IPv4 Routing
H3C S7500E Series Ethernet Switches Chapter 4 IS-IS Configuration
4-4
4.1.2 IS-IS Area
I. Two-level hierarchy
IS-IS uses two-level hierarchy in the routing domain to support large scale routing
networks. A large routing domain is divided into multiple Areas. The Level-1 router is in
charge of forwarding routes within an area, and the Level-2 router is in charge of
forwarding routes between areas.
II. 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.
 Note:
z The Level-1 routers in different areas can not establish the neighbor relationship.
z The neighbor relationship establishment of Level-2 routers has nothing to do with
area.
Figure 4-2 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.