3Com S7906E Configuration Manual page 713

End system (ES). An ES refers to a host system in TCP/IP. ISO defines the ES-IS protocol for
communication between an ES and an IS, and therefore an ES does not participate in the IS-IS
processing.
Routing domain (RD). A group of ISs exchanges routing information with each other using the
same routing protocol in a routing domain.
Area. An area is a unit in a routing domain. The IS-IS protocol allows a routing domain to be divided
into multiple areas.
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 the SPF algorithm and LSDB to generate its own routes.
Link State Protocol Data Unit (LSPDU) or Link State Packet (LSP). Each IS can generate an LSP
which contains all the link state information of the IS.
Network Protocol Data Unit (NPDU). An NPDU is a network layer protocol packet in OSI, which is
equivalent to an IP packet in TCP/IP.
Designated IS. On a broadcast network, the designated router is also known as the designated IS.
Network service access point (NSAP). An NSAP is an OSI network layer address. It identifies an
abstract network service access point and describes the network address in the OSI reference
model.
IS-IS address format
1) NSAP
As shown in Figure 1-1, an NSAP address consists of the Initial Domain Part (IDP) and the Domain
Specific Part (DSP). The IDP is equal to the network ID of an IP address, and the DSP is equal to the
subnet and host ID.
The IDP includes the Authority and Format Identifier (AFI) and the Initial Domain Identifier (IDI).
The DSP includes the High Order Part of DSP (HO-DSP), System ID and SEL, where the HO-DSP
identifies the area, the System ID identifies the host, and the SEL identifies the type of service.
The IDP and DSP are variable in length. The length of an NSAP address varies from 8 bytes to 20
bytes.
Figure 1-1 NSAP address format
2) Area address
The area address comprises 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
A system ID identifies a host or router uniquely. It has a fixed length of 48 bits (6 bytes).
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