3Com Switch 7757, Configuration Manual

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328 C
HAPTER
35: IS-IS C
ONFIGURATION
IS-IS network types
IS-IS supports two network types:
■
Broadcast networks, such as Ethernet and Token-Ring
■
Point-to-point networks, such as PPP and HDLC
For non-broadcast multi-access (NBMA) network, such as ATM, you need to
configure point-to-point or broadcast network on its sub-interfaces. IS-IS does not
run on point to multipoint (P2MP) links.
IS-IS Domain (Area) Two-level hierarchy
The 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.
Level-1 and Level-2
1 Level-1 router
The Level-1 router only forms 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 forms the neighbor relationship with the Level-2 and Level-1-2
routers in the same or in different areas. It maintains a Level-2 LSDB which
contains routing information for routing between areas. All Level-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 functions as a Level-1 and a Level-2 router is called a Level-1-2 router. It
can form the Level-1 neighbor relationships with the Level-1 and Level-1-2 routers
in the same area, or form Level-2 neighbor relationships with the Level-2 and
Level-1-2 routers in the same area or in different areas. A Level-1 router can be
connected to other areas only 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
Level-1 routers in different areas cannot form neighbor relationships.
Level-2 routers in different areas can form neighbor relationships.
Figure 76 shows a network topology running the IS-IS protocol. Area 1 is a set of
the Level-2 routers, called backbone network. The other four areas are
non-backbone networks connected to the backbone through Level-1-2 routers.