H3C Technologies H3C S10500 Series Switches - Manual (Page 151)

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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.
Typically, 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, ensure their system IDs are the same.
IS-IS area
Two-level hierarchy
IS-IS has a two-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
1. 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.
2. 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.
3. 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.
NOTE:
•
The Level-1 routers in different areas cannot establish neighbor relationships.
• The neighbor relationship establishment of Level-2 routers has nothing to do with area.
Figure 51 shows an IS-IS network topology. 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.