Operation Manual – IPv4 Routing
H3C S3610&S5510 Series Ethernet Switches
Chapter 4 IS-IS Configuration
4-3
Figure 4-1
NSAP address structure
2) Area
address
The area address is composed of the IDP and the HODSP of the DSP, which identify
the area and the routing domain. Different routing domains cannot have the same area
address.
Generally, a router only needs one area address, and all nodes in the same routing
domain must share the same area address. However, a router can have three area
addresses at most to support smooth area merging, partitioning and switching.
3) System
ID
The system ID identifies the host or router uniquely. It has a fixed length of 48 bits.
The system ID is used in cooperation with the Router ID in practical. For example, a
router uses the IP address 168.10.1.1 of the Loopback 0 as the Router ID, the system
ID in IS-IS can be obtained in the following way:
z
Extend each decimal number of the IP address to 3 digits by adding 0s from the
left, like 168.010.001.001;
z
Divide the extended IP address into 3 sections with 4 digits in each section to get
the System ID 1680.1000.1001.
There are other methods to define a system ID. Just make sure it can uniquely identify
a host or router.
4) SEL
The NSAP Selector (SEL), sometimes present in N-SEL, is similar with the protocol
identifier in IP. Different transport layer protocols use different SELs. All SELs in IP are
00.
5) Routing
method
Since the area is explicitly defined in the address structure, the Level-1 router can
easily recognize the packets sent out of the area. These packets are forwarded to the
Level-2 router.
The Level-1 router makes routing decisions based on the system ID. If the destination
is not in the area, the packet is forwarded to the nearest Level-1-2 router.
The Level-2 router routes packets across areas according to the area address.