H3C S7500 Series Operation Manual page 354

Operation Manual – Routing Protocol
H3C S7500 Series Ethernet Switches
IDP
AFI
Figure 5-3 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. This is equal to the area number in OSPF.
In normal condition, a router only needs one area address, and all nodes must share
the same area addresses in the same domain.
3) System ID
A system ID identifies a host or router uniquely. Its length has multiple options. For
S7500 series Ethernet switches, the length of the system ID is 48 bits (6 bytes).
The system ID is used together with the Router ID in practice. For example, a router
uses the IP address 168.10.1.1 of the Loopback 0 as the Router ID, you can get the
system ID used in IS-IS though the following method:
Extend each field of the IP address to 3 digits with putting 0s from the left;
Divide the extended IP address into 3 sections with 4 digits in each section, so
the System ID is 1680.1000.1001.
There are other methods to define a system ID. Just make sure it can uniquely identify
the host or router.
4) SEL
The NSAP Selector (SEL), sometimes present in N-SEL, is used as the protocol
identifier in IP. Different transmission protocols use different SELs. All SELs in IP are
00.
Because the area is explicitly defined in the address structure, the Level-1 router can
easily recognize the packets sent out of the area. Those 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.
Level-2 routers perform routing between areas according to the area address
(IDP+HO-DSP).
II. NET
The network entity title (NET) is an NSAP with SEL of 0. It indicates the network layer
information of the IS itself. SEL=0 means it provides no transport layer information.
In normal condition, a router only needs one NET. But a router can have three NETs
at most for smooth area merging and partitioning.
IDI High order DSP
Area address
DSP
System ID (6 octet)
5-5
Chapter 5 IS-IS Configuration
SEL (1 octet)