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NET
A network entity title (NET) indicates the network layer information of an IS, and does not include
transport layer information. It is a special NSAP address with the SEL being 0. The length of the NET is
equal to the NSAP, and is in the range of 8 bytes to 20 bytes.
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
•
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.
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.
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.
