Information Technology Reference
In-Depth Information
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.
Figure12-1 ISO-IGRP NSAP Addressing Structure
IDP
DSP
AFI
IDI
Area
System ID
S
1
Variable
ISO-IGRP
domain address
2
6
1
Area
address
System
address
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.
