Huawei Quidway S6500 Series Operation Manual page 204

Operation Manual - Routing Protocol
Quidway S6500 Series Ethernet Switches
Level-1/Level-2 router contiguous with a router in some other area will notify the
Level-1 routers in the local area that it has the exit of the area.
For the NPDU from an area destined to somewhere in another area, Level-1 router will
first transmit it to the nearest Level-2 router in the local area, regardless of its actual
destination area. And then the NPDU will be transmitted over the Level-2 backbone
network to a Level-1 router in the destination area. Finally the Level-1 router transmits
the NPDU to the destination.
Figure 5-1 illustrates a network running IS-IS routing protocol and composed of two
RDs, Routing Domain 1 and Routing Domain 2. Routing Domain 1 includes two areas,
Area 1 and Area 2, and Routing Domain 2 only has Area 3. In Routing Domain 1, the
three ISs connected by bold lines compose the area backbone. They are all Level-2
routers. The other 4 ISs not connected by bold line are Level-1 routers.
5.1.3 NSAP Structure of IS-IS Routing Protocol
The following figure illustrates the NSAP structure. The whole address is of 8 to 20
bytes long.
IDP
AFI IDI
Area Address (variable length)
Figure 5-2 NSAP structure
NSAP includes initial domain part (IDP) and domain specific part (DSP). IDP and DSP
are length-variable with total length of 20 bytes. IDP, occupying a byte, is composed of
address and format identifier (AFI) and initial domain identifier (IDI). AFI defines the
format of IDI. DSP has several bytes.
Area Address is composed of routing field and area identifier. The routing field includes
AFI and IDI and may also includes the first byte of DSP. It identifies the organization
structure. It is followed by an area identifier of 16 bits.
The following 48 bits (or 6 bytes) of System ID identifies the host or router uniquely. The
router belonging to different areas has only one identifier.
DSP
HO-DSP
System ID (6 octets)
Huawei Technologies Proprietary
5-3
Chapter 5 Integrated IS-IS Configuration
SEL 1 octet