IS-IS System and Area IDs:
In contrast to the OSPF AID and RID, which are expressed separately, the IS-IS AID and SysID are specified together in the Network Entity Title (NET). The NET is a special version of an ISO network service access point (NSAP) address, familiar to anyone who has worked with ISO protocols or with ATM. Figure 4.8 shows the basic format of a NET.
Figure 4.8. The format of a Network Entity Title.
As Figure 4.8 indicates, there are a few rules for configuring a NET:
- The AFI must be 1 byte.
- The remaining Area ID can be from 0 to 12 bytes.
- The SysID must be 6 bytes.
- The SEL must be 1 byte.
The NET is always specified in hexadecimal.
The Authority and Format Identifier (AFI) is actually a part of the Area ID, but is identified separately because of its special configuration rule. In ISO addresses, the AFI identifies the assigning authority of the address and the format of most of the rest of the address. But when the NET is assigned to a router in an IP-only network, the AFI has no real meaning separate from the rest of the AID.
The last byte, the NSAP Selector (SEL), is used in ISO protocols to identify an upper-layer function to which the address pointssomething like a port number in IP protocols. The SEL value 0x00 specifies the router itself. In an IP-only network, where there are no upper ISO protocol layers, a router never examines the SEL, which therefore can be set to any 1-byte value. Nonetheless, common practice is to always set the SEL to 0x00.