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encap HDLC vs. HDLC in PPP

Aug 18, 2012 8:31 PM

rosheuvel 7 posts since
Mar 4, 2009

I'm studying CCENT WANs and am a bit confused about HDLC:

there are 3 encapsulation choices: hdlc, ppp and frame-relay.

However, when I look at the PPP stack, it consists of HDLC, LCP and NCP protocols.

How can HDLC be a part of PPP, if PPP is a different encapsulation method than HDLC?

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    1. Aug 18, 2012 9:21 PM (in response to rosheuvel)
    Re: encap HDLC vs. HDLC in PPP

    Can you please post the SHOW output that you are referring to.  PPP is not HDLC but is based on it.  Ive never seen output where PPP and HDLC show up together.

     

    Kevin

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    3. Aug 18, 2012 9:39 PM (in response to rosheuvel)
    Re: encap HDLC vs. HDLC in PPP

    the image is not showing.

     

    Anyway, hdlc is cisco proprietary, and doesnt have the options and features that PPP has.

     

    If you enable ppp with authentication between two routers, then run debug ppp auth and debug ppp neg you will see it invoke the LCP and NCP - they are PPP related protocols

     

    HDLC does not invoke them, as its a much less sophisticated protocol.

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    4. Aug 18, 2012 9:45 PM (in response to rosheuvel)
    Re: encap HDLC vs. HDLC in PPP

    ok i see your other image now

     

    yes, PPP and HDLC are both considered Layer 2 protocols - they should have added PPP in that layer 2 column

     

    dont confuse them just because they run at the same OSI layer (just like TCP and UDP operate at layer 4, but they are different

    protocols)

     

    HTH Kevin

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    6. Aug 18, 2012 10:54 PM (in response to rosheuvel)
    Re: encap HDLC vs. HDLC in PPP

    its just a wording thing...PPP, HDLC, NCP, LCP are all layer 2 protocols, thats the important thing to remember.  PPP uses LCP and NCP; HDLC does not.

     

    For ccna thats about all you need to know, plus frame relay.

     

    As I mentioned PPP was created about 20 years ago or so, as an alternative to older Point to Point protocols like hdlc and slip that were proprietary and/or lacking in features, and was in a large degree based on HDLC.  Even the the packet structure of PPP looks a lot like HDLC.

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    8. Aug 18, 2012 11:07 PM (in response to rosheuvel)
    Re: encap HDLC vs. HDLC in PPP

    Dont get too hung up on some of the terminology...it gets tossed around pretty loosely sometimes.

     

    Are LCP and NCP sub-protocols of the "ppp stack"?  Technically, I have no  idea. Maybe.  For ccna you just need to know how PPP uses them (authentication).

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    9. Aug 18, 2012 11:12 PM (in response to rosheuvel)
    Re: encap HDLC vs. HDLC in PPP

    no probs,  layer 2 is fun, as it is relatively easy to understand.

     

    just type in "ppp compared with hdlc" in google and you will find a lot of pages that have rreally good info. with a lot of the history behind how they came about.

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  • Brian 2,971 posts since
    Aug 17, 2009
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    10. Aug 18, 2012 11:10 PM (in response to just plain old Kev)
    Re: encap HDLC vs. HDLC in PPP

    No HDLC is not Cisco proprietary.  However, Cisco's version of HDLC "is" proprietary.

     

    Here is a brief history of HDLC (source: WIKIPEDIA)

     

    History
    HDLC is based on IBM's SDLC protocol, which is the layer 2 protocol for IBM's Systems Network Architecture (SNA). It was extended and standardized by the ITU as LAP, while ANSI named their essentially identical version ADCCP.

     

    Derivatives have since appeared in innumerable standards. It was adopted into the X.25 protocol stack as LAPB, into the V.42 protocol as LAPM, into the Frame Relay protocol stack as LAPF and into the ISDN protocol stack as LAPD.

     

    HDLC was the inspiration for the IEEE 802.2 LLC protocol, and it is the basis for the framing mechanism used with the PPP on synchronous lines, as used by many servers to connect to a WAN, most commonly the Internet.

     

    A mildly different version is also used as the control channel for E-carrier (E1) and SONET multichannel telephone lines. Some vendors, such as Cisco, implemented protocols such as Cisco HDLC that used the low-level HDLC framing techniques but added a protocol field to the standard HDLC header. More importantly, HDLC is the default encapsulation for serial interfaces on Cisco routers. It has also been used on Tellabs DXX for destination of Trunk.

     

    In regards to PPP, it was designed after the original HDLC specifications, but included many new features such as authentication, encyrption and compression.

    PPP over serial links is encapsulated at the lower layer in HDLC type frames.  This is most likely why they show HDLC as part of the PPP protocol stack.

     

    Here are just some of the RFC's that cover PPP.

     

    RFC 1661 - Standard 51, The Point-to-Point Protocol (PPP)

    RFC 1662 - Standard 51, PPP in HDLC-like Framing

    RFC 1963 - PPP Serial Data transport Protocol

    RFC 2687 - Proposed Standard, PPP in a Real-time Oriented HDLC-like Framing


    Hope this helps.

     

    Brian

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    11. Aug 18, 2012 11:37 PM (in response to Brian)
    Re: encap HDLC vs. HDLC in PPP

    Brian wrote:

     

    No HDLC is not Cisco proprietary.  However, Cisco's version of HDLC "is" proprietary.



    absolutely correct, good clarification...was just speaking in terms of the ccna objectives

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  • Brian 2,971 posts since
    Aug 17, 2009
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    12. Aug 18, 2012 11:40 PM (in response to just plain old Kev)
    Re: encap HDLC vs. HDLC in PPP

    Understand.  Good posts by the way, short sweet and to the point.  Keep up the good work.

     

    Brian

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  • Darrel 3 posts since
    Feb 24, 2012
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    14. Nov 3, 2012 12:23 AM (in response to rosheuvel)
    Re: encap HDLC vs. HDLC in PPP

    In regrads to LCP and NCP.  It is my understanding that LCP will establish the link with the option of authentication. Once that is complete then NCP will negotiate L3 protocols to be carreid over the link? In essence what part of PPP negotiates the L3 protocols to be carreid over the link?

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