Network service access point address

IS-IS (ISO/IEC 10589) is implemented with network service access point (NSAP) addresses consisting of three fields: area address, system ID, and N-selector byte: service identifier (NSEL). The area address field may be from one to thirteen octets, the system ID field is normally six octets (must be six for Cisco IOS); and the NSEL identifies a process on the device. It is a loose equivalent to a port or socket in IP, the NSEL is not used in routing decisions. When the NSEL is set to 00, the NSAP is referred to as the network entity title (NET). NETs and NSAPs are represented in hexadecimal and must start and end on a byte boundary, such as 49.0001.1111.1111.1111.00 Level 1, or L1, IS-IS routing is based upon system ID. Therefore, each router must have a unique system ID within the area. L1 IS-IS routing equates to intra-area routing. It is customary to use either a MAC address from the router or, for Integrated IS-IS, to code the IP address, of a Loopback address, for example, into the system ID. However, numbering 1, 2, 3, 4, and so on is also acceptable. Area addresses starting with a value of 48, 49, 50, or 51 specify private addresses. This group of addresses should not be advertised to other connectionless network service (CLNS) networks. The area address must be the same for all routers in an area. On a LAN, one of the routers will be elected the designated intermediate system (DIS) based on interface priority. The default is 64. If all interface priorities are the same, the router with the highest subnetwork point of attachment (SNPA) address is selected. The (Ethernet) MAC addresses serves as the SNPA addresses for Ethernet LANs. The DIS serves the same purpose for IS-IS as the designated router (DR) does for OSPF. The ITA network engineer decides that SanJose1 will be the DIS, so its priority must be set higher than the default setting on the SanJose2 and SanJose3 routers.