3Com Switch 4800G 24-Port Configuration Manual page 327

The system ID is used in cooperation with the Router ID in practical. For example, a
router uses the IP address 168.10.1.1 of the Loopback 0 as the Router ID, the
system ID in IS-IS can be obtained in the following way:
Extend each decimal number of the IP address to 3 digits by adding 0s from the
■
left, like 168.010.001.001;
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. Just make sure it can uniquely
identify a host or router.
4 SEL
The NSAP Selector (SEL), sometimes present in N-SEL, is similar with the protocol
identifier in IP. Different transport layer protocols use different SELs. All SELs in IP
are 00.
5 Routing method
Since the area is explicitly defined in the address structure, the Level-1 router can
easily recognize the packets sent out of the area. These packets are forwarded to
the Level-2 router.
The 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.
The Level-2 router routes packets across areas according to the area address.
NET
The Network Entity Title (NET) is an NSAP with SEL of 0. It indicates the network
layer information of the IS itself, where SEL=0 means no transport layer
information. Therefore, the length of NET is equal to NSAP, in the range 8 bytes to
20 bytes.
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.
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 uses two-level hierarchy in the routing domain to support large scale routing
networks. A large routing domain is divided into multiple Areas. The Level-1 router
is in charge of forwarding routes within an area, and the Level-2 router is in
charge of forwarding routes between areas.
IS-IS Overview 327