3Com MSR 50 Series Configuration Manual page 878

878 C 57: IS-IS C
HAPTER ONFIGURATION
Link State Protocol Data Unit (LSP). Each IS can generate an LSP which contains
■
all the link state information of the IS. Each IS collects all the LSPs in the local
area to generate its own LSDB.
Network Protocol Data Unit (NPDU). An NPDU is a network layer protocol
■
packet in ISO, which is equivalent to an IP packet in TCP/IP.
Designated IS. On a broadcast network, the designated intermediate system is
■
also known as the designated IS or a pseudonode.
Network service access point (NSAP). The NSAP is the ISO network layer
■
address. It identifies an abstract network service access point and describes the
network address in the ISO reference model.
IS-IS address structure
1 NSAP
As shown in Figure 253, the NSAP address consists of the Initial Domain Part (IDP)
and the Domain Specific Part (DSP). The IDP is equal to the network ID of the IP
address, and the DSP is equal to the subnet and host IDs.
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), 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 253 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. Different routing domains cannot have the same
area address.
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
The system ID identifies the host or router uniquely. It has a fixed length of 48 bits
(6 bytes).
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:
DSP
System ID (6 octet) SEL (1 octet)