HP VPN Firewall Appliances Appendix Protocol Reference

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The DSP includes the high order part of DSP (HO-DSP), System ID, and SEL, where the HO-DSP identifies
the area, the System ID identifies the host, and the SEL identifies the type of service.
The IDP and DSP are variable in length. The length of an NSAP address varies from 8 bytes to 20 bytes.
Figure 10 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:
1. Extend each decimal number of the IP address to three 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 four digits in each section to get
the system ID 1680.1000.1001.
If you use other methods to define a system ID, make sure 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.