H3C Technologies H3C SR8800 - Manual (Page 154)

138
• 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 three NETs at most for smooth area merging and
partitioning. When you configure multiple NETs, make sure 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
• 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 which contains 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.
NOTE:
•
The Level-1 routers in different areas can not establish neighbor relationships.
• The neighbor relationship establishment of Level-2 routers has nothing to do with area.
Figure 50 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.