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7.2 | ![]() |
ISO Addressing | |
7.2.1 | ![]() |
NSAPs |
OSI network layer addressing is
implemented by using two types of hierarchical addresses, NSAP and
NET. The NSAP is a conceptual point on the boundary between the network and the transport layers. The NSAP is the location at which OSI network services are provided to the transport layer. Each transport layer entity is assigned a single NSAP. The NSAP address identifies any system in an OSI network. Various NSAP formats are used for various systems. Different protocols may use different representations of the NSAP.
The NSAP address is the network layer address for CLNS packets. As
with DECnet Phase V, one NSAP address is used for each device, not for
each interface.
An NSAP address consists of three parts.
They are the area address, the system
ID, and the NSAP selector byte. The total length is between eight and 20
bytes.
Cisco routers can route CLNS data that uses addressing conforming to
the ISO 10589 standard. The fields specified in this standard are as
follows:
IS-IS uses a simple two-layer architecture. IS-IS joins the IDP and HODSP together and treats them as the Level 2 area ID, with the remaining system ID used for Level 1 routing. Restated, in IS-IS, everything to the left of the system ID is used as the area ID. The minimum length of this area ID is a single byte. The maximum is the remaining 13 bytes permitted by the ISO standard. Therefore, an NSAP for an IS-IS network could be as little as eight bytes in length. The length is normally longer so as to permit some granularity in the allocation of areas.
There are three NSAP formats.
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