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NSAP Addresses

Since IS-IS was built on CLNS, it requires CLNS addressing.
CLNS addresses used by IS-IS apply to the entire device.
NSAP - Network Service Access Point
NSAP Address is up to 20 Bytes
Last byte (NSEL) is always 00 (Representing an IS), address is referred to as a NET number

ISO NSAP Address Format

  • IDP - Initial Domain Part
    • AFI - Authority and Format Identifier
    • IDI - Initial Domain Identifier - subdomain under the AFI
  • DSP - Domain Specific Part
    • High-Order DSP - Subdivides the domain into areas.
    • System ID - Identifies an individual OSI Device
    • NSEL - Identifies the process on the device

Cisco NSAP Address Format

IS-IS only uses three of the five OSI-defined fields
  • AFI is set to 49 - private address space (RFC 1918)
  • Area Address - Area ID
  • System ID - 6 bytes
  • NSAP Selector (NSEL) - set to 00
 
Example: 49.1234.aa15.b322.1841.00
49.1234 = Area
aa15.b322.1841 = System ID
00 = NSEL
 
Best practice is to ensure all system IDs are unique in the domain, but only have to be unique in the area.