H3C Technologies H3C S7500E Series Switches - Manual (Page 148)

5-3
Generally, a router only needs one area address, and all nodes in the same routing domain must share
the same area address. However, a router can have three area addresses at most to support smooth
area merging, partitioning and switching.
3) System ID
A system ID identifies a host or router uniquely. It has a fixed length of 48 bits (6 bytes).
The system ID of a device can be generated from the Router ID. For example, a router uses the IP
address 168.10.1.1 of Loopback 0 as the Router ID, and the system ID in IS-IS can be obtained in the
following way:
z Extend each decimal number of the IP address to 3 digits by adding 0s from the left, like
168.010.001.001;
z Divide the extended IP address into 3 sections with 4 digits in each section to get the system ID
1680.1000.1001.
There are other methods to define a system ID. The principle is to make sure it can uniquely identify a
host or router.
4) SEL
The NSAP Selector (SEL), or the N-SEL, is similar to the protocol identifier in IP. Different transport
layer protocols correspond to different SELs. All SELs in IP are 00.
5) Routing method
Because the area information is identified in IS-IS addresses, a Level-1 router can easily identify
packets destined to other areas.
z 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.
z A Level-2 router routes packets across areas according to the area address.
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. Therefore, the length of
the NET is equal to the NSAP and is in the range 8 bytes to 20 bytes.
A NET comprises three parts:
z Area ID. Its length is in the range of 1 to 13 bytes.
z System ID. A system ID uniquely identifies a host or router in the area and has a fixed 6-byte
length.
z 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 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.