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:


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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