3Com 7757 Configuration Manual page 330

330 C 35: IS-IS C
HAPTER ONFIGURATION
IS-IS Address Structure Address structure
1 NSAP
As shown in Figure 78, an NSAP address consists of the initial domain part (IDP)
and the domain specific part (DSP). The IDP is equal to the network id field in the
IP address, and the DSP is equal to the subnet and host id field.
The IDP, defined by ISO, includes the authority and format identifier (AFI) and the
initial domain identifier (IDI).
The DSP includes the high order DSP (HODSP), 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 lengths of IDP and DSP are variable. The length of an NSAP address varies
from 8 bytes to 20 bytes.
Figure 78 NSAP address structure
IDP
AFI IDI High order DSP
Area address
2 Area address
The area address is composed of the IDP and the HODSP of the DSP, which identify
the area and the 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.
3 System ID
The system ID identifies the host or router uniquely. The Switch 7750 implement 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.
4 SEL
DSP
System ID (6 octet) SEL (1 octet)