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1954 Views 11 Replies Latest reply: Apr 26, 2012 2:13 AM by Stephen RSS

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PPPoE vs Ethernet (802.3 & 802.2)

Apr 13, 2012 10:17 PM

KayouMT-NET 64 posts since
Aug 17, 2011

Hi,


I'm trying to understand how PPPoE is implemented and this raises some (may be out of CCNA) questions to me. But, I really want to understand how this protocol works. I did, unfortunately, not find practical tutorials explaining that subject.

 

Let's take the example of a home internet user connected via an ISP. I did understand that the telco local loop plays (kind-of) the role of a LAN segment connecting the user's PC (PPPoE client) to a PPPoE server (hosted to the ISP's office). If I'm right, my two questions are :


1) Is the local loop (of a DSL or Cable) compatible with Ethernet protocols 802.2 and 802.3 ; in terms, for example, of cable type, cable length, etc. ? Or, are some physical or logical adaptations done on the local loop for making it ready for Ethernet ?


2) Is the MAC address to which user's PC send data the PPPoE server's MAC address ?


Thanks in advance for any help !

  • Jon 13 posts since
    Apr 12, 2012
    Currently Being Moderated
    1. Apr 18, 2012 3:49 AM (in response to KayouMT-NET)
    Re: PPPoE vs Ethernet (802.3 & 802.2)

    Great questions!  I would also like to know the answers

  • Cristian F. Stoica 361 posts since
    Aug 7, 2011
    Currently Being Moderated
    2. Apr 18, 2012 6:30 AM (in response to KayouMT-NET)
    Re: PPPoE vs Ethernet (802.3 & 802.2)
  • chemilio 243 posts since
    May 31, 2009
    Currently Being Moderated
    3. Apr 18, 2012 8:48 AM (in response to KayouMT-NET)
    Re: PPPoE vs Ethernet (802.3 & 802.2)

    Hi KayouMT-NET,

     

    Lets look at the ethernet frame.

     

    http://www.ietf.org/rfc/rfc2516.txt

    Section 4. Payload  

    An Ethernet frame is as follows:

     

                                           1

                       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5

                      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      |       DESTINATION_ADDR |

                      |          (6 octets)           |

                      |                               |

                      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      | SOURCE_ADDR |

                      |          (6 octets)           |

                      |                               |

                      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      |    ETHER_TYPE  (2 octets)     |

                      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      ~                               ~

                      ~           payload             ~

                      ~                               ~

                      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      |           CHECKSUM            |

                      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     

     

     

    Where:

     

    DESTINATION_ADDR = unicast Ethernet destination address, Ethernet broadcast address (0xffffffff)

     

    SOURCE_ADDR=  The SOURCE_ADDR field MUST contains the _Ethernet MAC address_ of the _source device_


    To answer your 2nd question:

     

    2) Is the MAC address to which user's PC send data the PPPoE server's MAC address ?

     

    According to the RFC,  during the discovery process if the ethernet frame originates from the host then the mac address field is that of the _source device_. In this case therefore the specific host.

     

    But you must realise the  whole point of the discovery process is the host trying to figure out the _the destination mac address_ of the concentrator without it  how will the host know which concentrator to establish a session with? So i guess once the session is established the ethernet frame will have destination mac address as well.

     

    Depending on the phase,  the destination IP address is the unicast address or broadcast address( of the concentrator/server).

     

    That being said, why not connect two routers, configure PPPoE,  use a sniffer(wireshark), turn debugs on and observe for yourself the whole process. Much more interesting.

     

     

    HTH,

     

    Chemillio.

     

    Message was edited by: chemilio

  • Stephen 488 posts since
    Apr 22, 2011
    Currently Being Moderated
    4. Apr 19, 2012 4:59 AM (in response to KayouMT-NET)
    Re: PPPoE vs Ethernet (802.3 & 802.2)

    So wierd, I know how dsl works, i know how pppoe, fttc, & every type of internet works.  I've actually setup a isp from scratch in real life too for work, but I can't answer your question lol.  Here is a link that might be useful:

    http://www.pcvr.nl/tcpip/link_lay.htm

  • Currently Being Moderated
    6. Apr 19, 2012 8:29 AM (in response to KayouMT-NET)
    Re: PPPoE vs Ethernet (802.3 & 802.2)

    Hi KayouMT-NET

     

    Simulating a basic PPPoE connection in GNS3 is not that hard to do, but the configuration can become confusing and daunting if you don't understand all of the moving parts.  All you need is 2 routers connected to each other with an ethernet connection.

     

    R1--fa0/0---------------fa0/0-R2

     

    The server side configuration will go something like this.

     

    1. Create a loopback interface and assign an IP address

    2. Create a bba group and give it a name

    3. Bind the virtual-template interface to the bba group

    4. Bind the bba group to a physical interface.

    5. Create a local uername and password

    6. Create a local address pool that the client will use to get an IP address

    7. Apply unnumbered interface to the virtual-template

    8. Apply authentication to the virtual-template interface

     

    On the client side of things we'll do the following

     

    1. Enable PPPoE on the physical interface towards the server

    2. Bind the dialer pool to the physical interface

    3. Create a dialer list profile

    4. Create a dialer interface.

    5. Change the encapsulation to PPP

    6. Reduce the MTU on the dialer to account for extra encapsulation and avoid fragmentatin problems.

    7. Bind the dialer group to the interface that will be referenced on the physical interface.

    8. Bind the dialer pool on the interface that references the dialer list

    9. Enable PPP authentication.

    10. Tell the client how to get the IP address.

     

    Here is the server side configuration

     

    username R2 password 0 CCNA

    !

    ip local pool PPPOEPOOL 10.1.1.1 10.1.1.2

    !

    bba-group pppoe CCNA

    virtual-template 1

    !

    interface FastEthernet0/0

    no ip address

    pppoe enable group CCNA

    !

    interface Virtual-Template1

    mtu 1492

    ip unnumbered Loopback0

    peer default ip address pool PPPOEPOOL

    ppp authentication chap

     

    Now for the client side configuration

     

    dialer-list 1 protocol ip permit

    !

    interface FastEthernet0/0

    no ip address

    pppoe enable group CCNA

    pppoe-client dial-pool-number 1

    !

    interface Dialer0

    mtu 1492

    ip address negotiated

    encapsulation ppp

    dialer pool 1

    dialer-group 1

    ppp chap password 0 CCNA

     

    This is definitely beyond the scope of the CCNA and I wouldn't worry about this at all right now, but since you were curious, I thought I'd share with you.

     

    Have fun!

  • chemilio 243 posts since
    May 31, 2009
    Currently Being Moderated
    7. Apr 19, 2012 8:40 AM (in response to KayouMT-NET)
    Re: PPPoE vs Ethernet (802.3 & 802.2)

    Hi KayouMT-NET,

     

    I don't know how you could simulate the L1 stuff . To simulate the pc you can use a loopback interface with /32.

     

    But  we are interested to simulate what is happening at L2(MAC address) and L3.

     

    I think if you follow what this guy has done with GN3 to set up PPPoE with two routers and then hook  wireshark to see whats happening

     

    http://gns3vault.com/HDLC-PPP/ppp-over-ethernet-pppoe.html

    http://youtu.be/vaWXeZHFXvI

     

    Lastly I admit this is beyond CCNA level so I am also still learning about this stuff.

     

    HTH

     

    Chemillio.

  • Stephen 488 posts since
    Apr 22, 2011
    Currently Being Moderated
    8. Apr 19, 2012 10:23 AM (in response to KayouMT-NET)
    Re: PPPoE vs Ethernet (802.3 & 802.2)

    Hi KayouMT,

     

    I've got a little config guide you can use to configure pppoe on my blog.  Might be helpful for testing.  See link below:

    http://ccieblog.co.uk/dsl/configuring-dsl

  • Stephen 488 posts since
    Apr 22, 2011
    Currently Being Moderated
    10. Apr 20, 2012 4:39 AM (in response to KayouMT-NET)
    Re: PPPoE vs Ethernet (802.3 & 802.2)

    Ok. In this first image shows most of the components.  The MDF and DSLAM are in your local exchange. There's usually an ATM circuit to the ISP's network.

    1.jpg

    The BRAS is usually the layer 2 access concetrator (LAC).  Take at look at the diagram below. Ignore the PSTN bit.  The person who made this diagram below tried to summarise the first diagram by just calling it the PSTN:

    2.jpg

    When the user hits the LAC, their realm (which will usually be something like test@thisismyrealm.com) will be checked. If it is legitimate, the LAC will build a l2tp tunnel to the LNS. The LNS will then check the CPE's username and password against a stored list of usernames/passwords in the radius server.  If there's a match, the user will get an IP address, and a session to the LNS. Once you can get to the ISPs LNS, the ISP will route your traffic out to their upstream provider, and out to the internet.

  • Stephen 488 posts since
    Apr 22, 2011
    Currently Being Moderated
    11. Apr 26, 2012 2:13 AM (in response to KayouMT-NET)
    Re: PPPoE vs Ethernet (802.3 & 802.2)

    Just wanted to add.  This is really an amazing document for describing dsl

     

    http://www.tebyan.net/index.aspx?pid=31159&BookID=22012&PageIndex=30&Language=3

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