NSAP addressing and IS-IS NET Explained

NSAP Addressing

A Network Service Access Point address (NSAP address), defined in ISO/IEC 8348, is an identifying label for a service access point (SAP) used in OSI networking.

Network Service Access Point (NSAP) address aka ISO address is the network-layer address for CLNS packets (Connectionless Network Service).

For analogy with IP world:

• NSAP addressing can be compared to IP addressing
• CLNS can be compared to IP protocol.

NSAP address composed of the Initial Domain Part (IDP) and the Domain Specific Part (DSP). 

• The IDP refer to to the network ID of an IP address

• DSP refer to the subnet and host ID.

The IDP include:

• AFI – Authority and Format Identifier (1-byte): The AFI has a binary value between 0 and 99:
  AFI specifies the organization authorized to assign addresses, and the format and length of the rest of the CLNP address. The AFI is always 8 bits.
  
  AFI value identifies the IDI and DSP format. AFI set to 49 indicates private address space.

• IDI– Initial Domain Identifier (variable length): identifies the “suborganization” or sub-domain under the parent AFI organization. The length of the IDI is dependent on the AFI.

The DSP Include:

• HO-DSP (High-Order of DSP), it’s be used to identify the Area.

• System ID (System Identifier) (6-bytes), Identifies the host, it can contain the MAC address of the host.

• SEL (NSAP Selector) (1-byte), Identifies the type of service, for router it’s always 00

The combination of [IDP, HO-DSP] identify both the routing domain and the area within the routing domain. Hence the combination [IDP, HO-DSP] is called the “Area Address“. All nodes within the area must have same Area address.

IS-IS NET and System ID

A Network Entity Title (NET) is an NSAP address with NSEL set to 0. The NSAP address for all routers (ISes) are set with NSEL equal to 0.

Each IS-IS instance has an associated network entity title (NET). The NET is comprised of the IS-IS system ID, which uniquely identifies this IS-IS instance in the area and the area ID.

For example, if the NET is 49.0004.1111.1111.1111.00:

• The area is ID 49.0004
• The system ID is 1111.1111.1111
• NSEL: 00

Please refer to this article for IS-IS protocol basics:

Reference:

Routeralley

Knowledge Base

Cisco

Wikipedia