IS-IS Mechanics – CLNP

1. ISO OSI Terminology

ISO OSI term TCP/IP Equivalent
End System Host
Intermediate System Router
Circuit Interface
Area Area
Domain Autonomous System

IS-IS = Intermediate System to Intermediate System
CLNP = Connection-Less Network Protocol = Layer 3 network protocol that is used to communicate between ESes. CLNP offers a CLNS (Connection-Less Network Service). CLNP uses NSAP (Network Service Access Point) addressing. An NSAP address is assigned to an entire node, not to individual interfaces.
SNPA = Sub-Network Point of Attachment = Layer 2 address
Local Circuit ID = When added to the ISIS protocol, each interface gets Local Circuit ID. Values: 1-255.

1.1 NSAP Format

The NSAP address has 2 parts, which can be split into smaller pieces:

  • IDP = Initial Domain Part
    • AFI = Authority and Format Identifier = 1 octet long (00-FF). It identifies the format of the rest of the address. Usually 49 is used, which means the format/authority is local – similar to private addressing
    • IFI = Initial Domain Identifier = variable length, can miss if used only inside a domain
  • DSP = Domain Specific Part
    • HO-DSP = High Order Domain Specific Part = variable length,identifies an area of the domain
    • System ID = Identifies the node in an area. Length can be between 1 and 8 Bytes. Usually 6 Bytes are used
    • SEL = Identifies the service that should process the packets on the node. If SEL=0, the frame is for the node itself and the NSAP address with SEL=0 is called NET = Network Entity Title
R(config-router)# net 49.0001.1111.1111.1111.00
! 49 = AFI
! 0001 = HO-DSP (Area)
! 1111.1111.1111.1111 = System ID
! 00 = SEL

2. Routing Levels

Level 0 = ES to ES on the same link (or ES to IS on the same link). ES and IS send Hellos – ESH and ISH – to advertise each other’s presence on the network.
Level 1 = ES to ES in a single area = IS nodes collect lists of ES nodes directly attached to them and exchange this information with all other IS in the area.
Level 2 = ES to ES in different areas from the same domain = IS nodes exchange area prefixes
Level 3 = ES to ES in different domains = Initially IDRP (Inter Domain Routing Protocol) was used to exchange domain information, but now BGP can be used since now it’s protocol independent.

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