3Com Switch 8800 Configuration Guide
Chapter 20 Integrated IS-IS Configuration
20-5
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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-2 router.
The Level-2 router performs intra-area routing according to the area address (IDP +
HO-DSP).
II. NET
Network Entity Title (NET) indicates the network layer information, which contains no
transfer layer information (SEL=0). You can regard it as a special NSAP.
In general, you can configure a NET for a router. If you will redivide an area (combine
multiple areas or divide an area into multiple areas), you can configure multiple NETs to
ensure correct routes in the case of reconfiguration. Because you can configure up to
three area addresses, you can only configure up to three NETs.
For example, there is a NET 47.0001.aaaa.bbbb.cccc.00, in which,
Area=47.0001, System ID=aaaa.bbbb.cccc, SEL=00.
For example, there is a NET 01.1111.2222.4444.00, in which,
Area=01, System ID=1111.2222.4444, and SEL=00.
20.1.4 IS-IS Routing Protocol Packets
IS-IS packets are directly encapsulated in the data link frames and mainly divided into 3
kinds, Hello, LSP and SNP.
I. Hello packets
Hello packets, which is also called IIH (IS-to-IS Hello PDUs), can establish and
maintain neighbor relations. The Level-1 router in a broadcast LAN forwards Level-1
LAN IIH; the Level-2 router in a broadcast LAN forwards Level-2 LAN IIH;
non-broadcast network forwards Point-to-Point IIH.
II. LSP
Link state packet (LSP) can switch link state information. LSP can be divided into
Level-1 LSP and Level-2 LSP. Level-2 routers transmit Level-2 LSPs; Level-1 routers
transmit Level-1 LSPs; Level-1-2 routers transmit both Level-2 LSPs and Level-1
LSPs.
