3Com 4800G Series, Configuration Manual

Page: 326 / 1246

326 C
HAPTER
29: IS-IS C
ONFIGURATION
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Link State Database (LSDB). All link states in the network forms the LSDB. There
is at least one LSDB in each IS. The IS uses SPF algorithm and LSDB to generate
its own routes.
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Link State Protocol Data Unit (LSPDU) or Link State Packet (LSP). Each IS can
generate a 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.
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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.
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Designated IS. On a broadcast network, the designated router is also known as
the designated IS or a pseudonode.
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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 99, 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 99 NSAP address structure
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).
AFI IDI High order DSP System ID (6 octet) SEL (1 octet)
IDP
DSP
Area address