IS-IS address format
NSAP
As shown in Figure 32, an NSAP address comprises the Initial Domain Part (IDP) and the Domain Specific Part (DSP). The IDP is analogous to the network ID of an IP address, and the DSP is analogous to the subnet and host ID.
The IDP includes the Authority and Format Identifier (AFI) and the Initial Domain Identifier (IDI).
The DSP includes:
High Order Part of DSP (HO-DSP)—Identifies the area.
System ID—Identifies the host.
SEL—Identifies the type of service.
The IDP and DSP are variable in length. The length of an NSAP address ranges from 8 bytes to 20 bytes.
Figure 32: NSAP address format
Area address
The area address comprises the IDP and the HO-DSP of the DSP, which identify the area and the routing domain. Different routing domains cannot have the same area address.
Typically, a router only needs one area address, and all nodes in the same area must have the same area address. To support smooth area merging, partitioning, and switching, a router can have a maximum of three area addresses.
System ID
A system ID uniquely identifies a host or router. It has a fixed length of 48 bits (6 bytes).
The system ID of a device can be generated from the router ID. For example, suppose a router uses the IP address 168.10.1.1 of Loopback 0 as the router ID. The system ID can be obtained in the following steps:
Extend each decimal number of the IP address to three digits by adding 0s from the left, such as 168.010.001.001.
Divide the extended IP address into three sections that each has four digits to get the system ID 1680.1000.1001.
If you use other methods to define a system ID, make sure that it can uniquely identify the host or router.
SEL
The N-SEL, or the NSAP selector (SEL), is similar to the protocol identifier in IP. Different transport layer protocols correspond to different SELs. All SELs in IP are 00.
Routing method
The IS-IS address format identifies the area, so a Level-1 router can easily identify packets destined to other areas. IS-IS routers perform routing as follows:
A Level-1 router performs intra-area routing according to the system ID. If the destination address of a packet does not belong to the local area, the Level-1 router forwards it to the nearest Level-1-2 router.
A Level-2 router performs inter-area routing according to the area address.