System information

ISO CLNS Technology Basics
Book Title
12-272
ISO CLNS Addressing
Addresses in the ISO network architecture are referred to as NSAP addresses and network entity
titles (NETs). Each node in an OSI network has one or more NETs. In addition, each node has many
NSAP addresses. Each NSAP address differs from one of the NETs for that node in only the last byte
(see Figure 12-1). This byte is called the n-selector. Its function is similar to the port number in other
protocol suites.
Cisco’s implementation supports all NSAP address formats that are defined by ISO 8348/Ad2;
however, Cisco provides dynamic routing (ISO-IGRP or IS-IS routing) only for NSAP addresses
that conform to the address constraints defined in the ISO standard for IS-IS (ISO 10589).
An NSAP address consists of two major fields:
• The initial domain part (IDP) is made up of 1-byte AFI and a variable-length initial domain
identifier (IDI). The length of the IDI and the encoding format for the domain-specific part (DSP)
are based on the value of the authority and format identifier (AFI).
• The DSP is made up of a high-order DSP, an area ID, a system ID, and a 1-byte n-selector.
The key difference between the ISO-IGRP and IS-IS NSAP addressing schemes is in the definition
of area addresses. Both use the system ID for Level 1 routing. However, they differ in the way
addresses are specified for area routing. An ISO-IGRP NSAP address includes three separate levels
for routing: the domain, area, and system ID. An IS-IS address includes two fields: a single
continuous area field comprising the domain and area fields defined for ISO-IGRP and the system
ID.
Figure 12-1 illustrates the ISO-IGRP NSAP addressing structure.
Figure 12-1 ISO-IGRP NSAP Addressing Structure
The ISO-IGRP NSAP address is divided into three parts: a domain part, an area address, and a
system ID. Domain routing is performed on the domain part of the address. Area routing for a given
domain uses the area address. System ID routing for a given area uses the system ID part. The NSAP
address is laid out as follows:
• The domain part is of variable length and comes before the area address.
• The area address is the 2 bytes before the system ID.
• The system ID is the 6 bytes before the n-selector.
• The n-selector (S) is the last byte of the NSAP address.
Our ISO-IGRP routing implementation interprets the bytes from the AFI up to (but not including)
the area field in the DSP as a domain identifier. The area field specifies the area, and the system ID
specifies the system.
Figure 12-2 illustrates the IS-IS NSAP addressing structure.
IDP DSP
AFI IDI Area System ID S
1 Variable
ISO-IGRP
domain address
Area
address
System
address
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