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IS-IS Network Design Solutions by Scott Sturgess, Abe Martey

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NSAP Selector Values

The NSEL field specifies a user of the network layer service. The routing layer is regarded as a special user of network layer services and is assigned a value of zero for the NSEL.

The NSAP configured on IS-IS routers always uses 0x00 for the NSEL; such addresses are also referred to as network entity titles (NETs). NSEL values have the same connotation as the Protocol Identifier in Layer 2 addresses or TCP port numbers. The NSEL value assists the network layer in handing off datagrams to the appropriate application or service user. According to the OSI layering scheme, the basic user of network layer services is the transport layer. CLNP data packets that are not meant for the routing process have target NSAP addresses ...

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