IS-IS Network Design Solutions

OSI Network Addresses

In the OSI architecture, the network layer provides services to a user in the layer above, such as the transport service element. If you designate the network layer by N, the network user is the N+1 service element (see Figure 4-2). An N+1 entity in Router A that wants to communicate with an N+1 peer in Router B must provide the peer's N service access point (NSAP) as the destination of the service request while providing its own NSAP as the source to facilitate bidirectional communication. In a router, the N+1 entity or the N service user is either a transport layer entity or the routing layer.

Figure 4-2. Peer-to-peer layer communications.

The NSAP address format has a component known as the NSAP selector (NSEL) for identifying ...