3Com Switch 7750 Configuration Guide Guide

316 CHAPTER 35: IS-IS CONFIGURATION
The DSP includes the high order DSP (HODSP), the System ID and SEL, where the
HODSP identifies the area, the System ID identifies the host, and the SEL indicates
the type of service.
The length of IDP and DSP is variable. The length of the NSAP address varies from
8 bytes to 20 bytes.
Figure 75 NSAP address structure
1 Area address
The area address is composed of the IDP and the HODSP of the DSP, which identify
the area and the routing domain. This is equal to the area number in OSPF. It is not
allowed to have same area addresses in the same routing domain.
In normal condition, a router only needs one area address, and all nodes must
share the same area addresses in the same domain. But a router can have three
area addresses at most to support smooth area merging, partitioning and
switching.
2 System ID
The system ID identifies the host or router uniquely. The Comware implements a
fixed length of 48 bits (6 bytes).
The system ID is used in cooperate 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, you can
get the system ID used in IS-IS though the following method:
■ Extend each field of the IP address to 3 digits with putting 0s from the left, like
168.010.001.001;
■ Divide the extended IP address into 3 sections with 4 digits in each section, so
the System ID is 1680.1000.1001.
There are other methods to define a system ID. Just make sure it can uniquely
identify the host or router.
3 SEL
The NSAP Selector (SEL), sometimes present in N-SEL, is used as the protocol
identifier in IP. Different transmission protocols use different SELs. All SELs in IP are
00.
Because the area is explicitly defined in the address structure, the Level-1 router
can easily recognize the packets sent out of the area. Those packets are forwarded
to the Level-2 router.
AFI IDI High order DSP System ID
SEL
(1 octet)
DS PIDP
Area address
AFI IDI High order DSP System ID
SEL
(1 octet)
DS PIDP
Area address