3com S7906E Manuel De Montage

 

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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. 

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Routing domain (RD). A group of ISs exchanges routing information with each other using the 

same routing protocol in a routing domain. 

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Area. An area is a unit in a routing domain. The IS-IS protocol allows a routing domain to be divided 

into multiple areas. 

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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 the 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 an LSP 

which contains all the link state information of the IS.  

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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. 

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Designated IS. On a broadcast network, the designated router is also known as the designated IS. 

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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. 

1) NSAP 

As shown in 

, 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).