R2511-HP MSR Router Series Layer 3 - IP Routing Configuration Guide(V5)

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The IDP and DSP are variable in length. The length of an NSAP address varies from 8 bytes to 20 bytes.
Figure 28 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 through the
following steps:
1. Extend each decimal number of the IP address to 3 digits by adding 0s from the left, such as
168.010.001.001.
2. Divide the extended IP address into three sections that each has 4 digits in each section 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 SEL, or the NSAP selector (N-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.
NET
A network entity title (NET) indicates the network layer information of an IS. It does not include transport
layer information. A NET is a special NSAP address with the SEL being 0. The length of a NET is in the
range of 8 bytes to 20 bytes, same as a NSAP address.
A NET comprises the following parts:
• Area ID—Has a length of 1 to 13 bytes.