NET

Stands for: Network Entity Title (the IS-IS router address, an NSAP with selector 00)

The OSI-style address every IS-IS router needs: an area address, a system ID and a selector of 00.

Even an IP-only router running IS-IS must carry one, and uses it only for IS-IS. It reads as the area, then a six-octet system ID that names the router in the domain, then the selector: 49.0001.1920.0000.2001.00 is a router in area 49.0001.

An NSAP address divides into an area address, a system identifier and a one-octet selector; a NET is an NSAP whose selector is 00. Juniper's documentation gives NETs a length of 8 to 20 octets, the simplest 10, with an area part of 1 to 13 octets. RFC 3719 says an implementation MUST use a system ID length of 6, which has proved ample because the ID matters only inside the routing domain.

The system ID is usually minted from something already unique. Juniper recommends an interface IP address in binary-coded decimal: 192.0.2.1, padded to 192.000.002.001, becomes 1920.0000.2001. RFC 1195 instead prepends hex 02 00 to the address, giving 0200.c000.0201, which cannot collide with an ID taken from a MAC address because 02 sets the bit that globally assigned MACs leave clear.

Every ID must be unique in the domain, and Juniper notes that an all-zero system ID is illegal and forms no adjacency. Level 1 routers refuse neighbours whose area addresses do not overlap their own, so a mistyped area shows up as a missing adjacency; FRRouting's show isis summary prints the NET and system ID in use.

Also known as: network entity title, nsap, system id, nsel, area address

Sources

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