H3C Technologies H3C S12500 Series Switches - Manual (Page 150)

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A NET comprises the following parts:
• Area ID—Its length is in the range of 1 to 13 bytes.
• System ID—A system ID uniquely identifies a host or router in the area and has a fixed 6-byte
length.
• SEL—It has a value of 0 and a fixed 1-byte length.
For example, a NET is ab.cdef.1234.5678.9abc.00, where area ID is ab.cdef, system ID is
1234.5678.9abc, and SEL is 00.
Generally, a router only needs one NET, but it can have a maximum of three NETs for smooth area
merging and partitioning. When you configure multiple NETs, make sure their system IDs are the same.
IS-IS area
IS-IS has a 2-level hierarchy to support large-scale networks. A large-scale routing domain is divided into
multiple areas. Typically, a Level-1 router is deployed within an area, a Level-2 router is deployed
between areas, and a Level-1-2 router is deployed between Level-1 and Level-2 routers.
Level-1 and Level-2
• Level-1 router—A Level-1 router establishes neighbor relationships 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 destined for an outside area to the nearest Level-1-2 router.
• Level-2 router—A Level-2 router establishes neighbor relationships with the Level-2 and Level-1-2
routers in the same or in different areas. It maintains a Level-2 LSDB containing inter-area routing
information. All the Level-2 and Level-1-2 routers must be contiguous to form the backbone of a
routing domain.
• Level-1-2 router—A router with both Level-1 and Level-2 router functions is a Level-1-2 router. It can
establish Level-1 neighbor relationships with the Level-1 and Level-1-2 routers in the same area, or
establish Level-2 neighbor relationships with the Level-2 and Level-1-2 routers in different areas. A
Level-1 router must be connected to other areas through 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.
Level-1 routers in different areas cannot establish neighbor relationships. Whether Level-2 routers
establish neighbor relationships does not depend on areas.
Figure 53 shows one IS-IS network topology, where Area 1 comprises a set of Level-2 routers and is the
backbone. The other four areas are non-backbone areas connected to the backbone through Level-1-2
routers.