1 2 Previous Next 15 Replies Latest reply: Apr 13, 2014 6:03 AM by vannaramao RSS

    encap HDLC vs. HDLC in PPP

    rosheuvel

      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?

        • 1. Re: encap HDLC vs. HDLC in PPP
          just plain old Kev

          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

          • 2. Re: encap HDLC vs. HDLC in PPP
            rosheuvel

            hello Kevin,

             

            for the encapsulation choices, I'm referring to the IOS help output (cut for clarity):

            Router(config-if)# encap ?

            frame-relay        Frame Relay networks

            hdlc                  Serial HDLC synchronous

            ppp                   Point-to-Point protocol

             

            for the PPP stack, I'm referring to the below:

            PPP Protocol stack compared to OSI Reference Model:

            OSI Layer
            PPP Protocol Stack
            3Upper-layer Protocols (such as IP, IPX, AppleTalk)
            2
            Network Control Protocol (NCP)
            Link Control Protocol (LCP)
            High-Level Data Link Control Protocol (HDLC)
            1Physical layer (such as EIA/TIA-232, V.24, V.35, ISDN)

             

            Thank you

            • 3. Re: encap HDLC vs. HDLC in PPP
              just plain old Kev

              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.

              • 4. Re: encap HDLC vs. HDLC in PPP
                just plain old Kev

                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

                • 5. Re: encap HDLC vs. HDLC in PPP
                  rosheuvel

                  Thanks for the quick reply.

                  My understanding (from the image) is that the PPP protocol stack is everything listed in the layer 2 section: HDLC, LCP and NCP, because the title of the image is "PPP protocol stack". Am I mis-understanding something? Or does PPP only consist of LCP and NCP?

                  (added color to initial image, for clarity)

                  • 6. Re: encap HDLC vs. HDLC in PPP
                    just plain old Kev

                    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.

                    • 7. Re: encap HDLC vs. HDLC in PPP
                      rosheuvel

                      Thank you, Kevin!

                      I think I'm getting the idea now; that was very helpful.

                      • 8. Re: encap HDLC vs. HDLC in PPP
                        just plain old Kev

                        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).

                        • 9. Re: encap HDLC vs. HDLC in PPP
                          just plain old Kev

                          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.

                          • 10. Re: encap HDLC vs. HDLC in PPP
                            Brian

                            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

                            • 11. Re: encap HDLC vs. HDLC in PPP
                              just plain old Kev

                              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

                              • 12. Re: encap HDLC vs. HDLC in PPP
                                Brian

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

                                 

                                Brian

                                 

                                • 13. Re: encap HDLC vs. HDLC in PPP
                                  rosheuvel

                                  "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"

                                  This looks like the final answer.

                                  Thank you so much, Kevin and Brian (for all the details) !

                                  • 14. Re: encap HDLC vs. HDLC in PPP
                                    Darrel

                                    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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