Operation Manual – IPv4 Routing
H3C S3610&S5510 Series Ethernet Switches
Chapter 4 IS-IS Configuration
4-4
III. NET
A Network Entity Title (NET) is an NSAP with SEL being 0. It indicates the network layer
information of the IS itself, with no transport layer information. Therefore, the length of
NET is equal to NSAP, in the range 8 bytes to 20 bytes.
Generally, a router only needs one NET, but it can have three NETs at most for smooth
area merging and partitioning. When you configure multiple NETs, make sure their
system IDs are the same.
For example, a NET is ab.cdef.1234.5678.9abc.00, where,
Area = ab.cdef, System ID = 1234.5678.9abc, and SEL = 00.
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 generally 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.
