IS-IS: NSAP Address Format

NSAP addresses use hexadecimal digits separated into groups of one or more octets by a dot.The use of a dot is arbitrary making the following example notations of an NSAP address represent the same address:

49.001.1234.5678.3333.00
4900.0112.3456.7833.3300
49.00.01.12.34.56.78.33.33.00
49000112.34.5678.33.3300
4900011234567833300

If any of these are configured Cisco will rewrite the address to the notation used on the first line.  NSAP addresses may be easier to read from right to left:

example NSAP: 49.0001.1234.5678.3333.00

SEL Value = 00
System ID = 1234.5678.3333
IDI = 0001
AFI = 49

When a packet is routed it is encapsulated into a frame addressed to the next directly attached hop identified by its layer 2 address.  In OSI the Layer 2 address of an interface is called a Sub Network Point of Attachment (SNPA)

To distinguish between interfaces on a node, the IS enumerates interfaces using a locally significant 1-octect number called the Local Circuit ID which increments by 1 for every interface added to the IS-IS instance beginning with 0 on Cisco routers.

 

 

 

 

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