3com 3031 Instruccion De Instalación

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SEL

The role of an SEL (NSAP Selector, or N-SEL) is similar to the “protocol ID” of IP. 
Different transportation protocols correspond to different SELs. The SEL over IP 
is "00”.

As this kind of address structure definitely defines an area, Level-1 routers can 
easily identify packets sent to an outside area, which packets are to be 
forwarded to Level-2 routers.

Level-1 routers use System ID to implement intra-area routing. Once a Level-1 
router finds that the destination address of a packet is out of its own area, it 
will forward the packet to a nearest Level-2 router.

Level-2 routers perform inter-area routing according to their area addresses 
(IDP + HO-DSP).

Network Entity Title (NET) indicates the network layer information of IS itself, 
excluding transport layer information (SEL = 0). It can be regarded as a special 
NSAP.

Generally, a router is only configured with one NET. When it is necessary to 
reconstruct an area, for example, to combine many areas together or to divide an 
area into separate areas, multiple NETs should be configured on a router so that 
route correctness can be guaranteed even upon reconfiguration.  A maximum of 
three NETs may be configured on an IS-IS router.

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, SEL = 00.

IS-IS packets are directly encapsulated in data link frames. Basically, they fall into 3 
categories: Hello packet, LSP and SNP.

Hello packets, also known as IIH (IS-to-IS Hello PDUs), are used to create and 
maintain peers. Among them, the Level-1 LAN IIH apply to Level-1 routers on 
broadcast LAN, Level-2 LAN IIH apply to Level-2 routers on broadcast LAN and 
Point-to-Point IIH apply to non-broadcast networks.

LSP (Link State PDUs) are used for exchanging link state information. There are 
two kinds of LSPs: Level-1 LSP and Level-2 LSP. Level-2 LSP is forwarded via Level-2 
routers; while Level-1 LSP is forwarded via Level-1 routers. Level-1-2 routers can 
forward both.

SNP (Sequence Number PDUs) are used to confirm the newly received LSP 
between neighbors. Their role is similar to, but more effective than Acknowledge 
packets. SNP include CSNP (Complete SNP) and PSNP (Partial SNP), which are 
further divided into Level-1 CSNP, Level-2 CSNP, Level-1 PSNP and Level-2 PSNP.