Yap Chin Hoong has been looking at the OSI protocol stack I’ve published and asked an interesting question: “where is CLNS in that protocol stack?

The OSI protocol stack has a major advantage over the TCP/IP stack: it defines both the protocols and the APIs between the layers. CLNS (Connection-less network Service) is the API (the function calls that allow transport layers to exchange datagrams across the network) while CLNP (Connection-less network Protocol) is the layer-3 protocol that implements CLNS. In my diagram, CLNS would be a thin line above CLNP between L3 and L4 boxes.

IOS developers did not escape the confusion between CLNS and CLNP. The clns routing command does not make sense; you cannot route an API. The command should have been called clnp routing.


  1. It means CLNS is like device driver.
  2. Hi Ivan, thanks for answering my question. :-)
    Kindly address me in full name, cuz my first name should be Chin Hoong instead, Yap is the surname. :-)
    I am still in the midst of studying the theory of IS-IS (yes it is obsolete in CCNP syllabus but I studying for my own good), I haven't spend many times in labbing IS-IS stuffs, I believe I will have better visualization when I capture the packets upon the "ping clns" command. :-)

    thanks and have a nice day... expect more questions from me in the future. 8-)
  3. Oh, my ... I apologize for the blunder. I'm used to the surname being last; another one of those culture-specific things that I'm totally unaware of. Thank you!

    When you do a capture on a LAN segment between two routers, you should see ES-IS packets (routers advertising themselves to non-existent hosts), IS-IS packets (obvious) as well as CLNP packets (when you do ping).
  4. Really thanks for the info, I spent quite some times today trying to figure out how to capture ES-IS packets, even thought of running OpenVMS and configuring DECnet. =-X
  5. Hey Guys,

    I was wondering if you could help me...
    I've been studying IS-IS because I'll be participating in a intense course in the mid August and I'm trying to be a bit proactive.


    I'm really trying to figure out CLNS's role with IS-IS and it seems like this topic is ambiguous and no one can give me a straight answer.

    I've read a book that says "OSI suite is also referred to as the CLNS suite", that "CLNS + CLNP = IP (in the TCP/IP stack)..

    You wrote: "In my diagram, CLNS would be a thin line above CLNP between L3 and L4 boxes."

    However, this site shows:
    that it's a thin line between layer 2 and 3.

    I need some help and if you guys could reply, that would be fantastic..

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