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 with non-zero NSEL values to indicate the network service user at the transport layer. This does not apply to IP packets that are routed based on an IP destination address. For example, the NSEL value of 0x21 identifies the transport layer of DECNet Phase IV and a value of 0x22 for OSI Transport Layer TP4. OSI TP4 is implemented in DECNet Phase V (see Table 4-2). The transport layer then ultimately hands off to a higher protocol layer, possibly to the end application.

Table 4-2. NSAP Selector (NSEL) Values

NSEL Value	Network Service User
0x00	Routing Layer (for instance, IS-IS)
0x21	DECNet Phase IV Transport Layer
0x22	OSI Transport Layer TP4

NSAP Examples

This section presents concrete examples of CLNP NSAP addresses (see Examples 4-8, 4-9, and 4-10) to help the reader precipitate material covered on CLNP addressing structure.

Example 4-8 A CLNP Example

 47.0001.aaaa.bbbb.cccc.00 Area = 47.0001, SysID = aaaa.bbbb.cccc, NSel = 00 

Example 4-9 A CLNP Example

 39.0f01.0002.0000.0c00.1111.00 Area = 39.0f01.0002, SysID = 0000.0c00.1111, NSel = 00 

Example 4-10 A CLNP Example

 49.0002.0000.0000 .0007.00 Area = 49.0002, SysID = 0000.0000.0007, Nsel = 00