169
The area information is identified in IS-IS addresses, so a Level-1 router can easily identify packets
destined to other areas.
•
A Level-1 router makes routing decisions based on the system ID. If the destination is not in
the area, the packet is forwarded to the nearest Level-1-2 router.
•
A Level-2 router routes packets across areas according to the area address.
NET
A 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; therefore, the length of the NET is
equal to the NSAP, and is in the range of 8 bytes to 20 bytes.
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, ensure 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.
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.