3Com 8807 Configuration Manual page 342

342 C 32: I
HAPTER NTEGRATED
IS-IS C
ONFIGURATION
OSI adopts the address structure as shown in Figure 88. NSAP includes initial
domain part (IDP) and domain specific part (DSP). The IDP is defined by ISO; it
consists of authority responsible for assigning the rest of the address and address
format. The DSP is allocated by the authority specified in IDP. IDP and DSP are
length-variable with a total length of 20 bytes.
Area Address
■
IDP includes authority and format identifier (AFI) and initial domain identifier (IDI).
AFI defines the format of IDI. DSP has several bytes. The combination of IDP and
HO-DSP can identify a route area and an area of the route area, so the
combination is called an area address.
In general, you only need to configure an area address for a router. The area
addresses of all nodes are the same in an area. To support the seamless
combination, segmentation and conversion, Switch 8800 Family series support up
to three area addresses.
System ID
■
System ID uniquely identifies terminal system or router in a route area. You can
select length for it. For Switch 8800 Family series, System ID length is 48 bits (6
bytes). In general, you can obtain System ID according to Router_ID.
If the IP address 168.10.1.1 of the interface LoopBack0 serves as a router_ID for
the router, you can use the following method to obtain the System ID:
Turn each part of the IP address 168.10.1.1 into three digits. Add 0 to the front of
the part less than three digits.
Divide the expanded address 168.010.001.001 into three parts. Each part contains
four digits.
You get the System ID 1680.1000.1001.
You can specify a System ID using different methods. However, you should ensure
a System ID can uniquely identify a terminal system or a router.
SEL
■
NSAP selector (SEL or N-SEL) functions as the protocol identifier of an IP address.
Different transmission protocols correspond to different identifiers. All the SELs of
IP are 00.
Because the address structure defines clearly an area, a Level-1 router can easily
identify the packets not sent to the area where it is located. The Level-1 router
forwards the packets to a Level-2 router.
The Level-1 router performs routing within areas by System IDs. If it detects the
destination address of a packet does not belong to the area where it is located, it
forwards the packet to its closest Level-1-2 router.
The Level-2 router performs intra-area routing according to the area address (IDP +
HO-DSP).