RFC1483 Bridged vs Bridged Mode Only

Discussion in 'Broadband' started by Simon Dean, May 21, 2006.

  1. Simon Dean

    Simon Dean Guest

    When connecting my ethernet modem to my smoothwall router/firewall,
    could someone please explain, on the modem, what the difference is
    between RFC1483 Bridged and Bridged Mode Only?

    Which ones can be used in the UK?

    You see, I keep coming back to the fact that in the UK, we're PPPoA, but
    when you want to connect your modem to your router, you can't specify
    PPPoA, it's RFC1483 Bridged, or Bridged Mode Only.

    In Bridged Mode Only, we get to use PPPoE to authenticate from our
    router, our Linux box, or our Windows XP, as suggested here:

    So then presumably, rather than connecting by PPPoA, we go by PPPoE
    because the computer takes control of the connection.

    I've seen references for RFC 1483 - Multiprotocol Encapsulation over ATM.

    So then I start to wonder, what does that mean? If we set up in RFC1483,
    will the modem convert PPPoE from the router to PPPoA?

    Try discussing this with Americans, and they can't quite understand PPPoA.

    Simon Dean, May 21, 2006
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  2. Simon Dean

    Christopher Guest

    Simon - this from a forum


    How can I tell if I am using PPPOE or RFC1483?

    PPPOE - If your ISP assigned you a username and password; you are using

    If your ISP assigned you a public static IP address, Subnet Mask, Default
    Gateway and primary and secondary DNS addresses; you are using RFC1483.

    Note: If you are not sure please contact your ISP.


    hope it helps
    Christopher, May 21, 2006
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  3. Simon Dean

    Moonshine Guest

    PPPoE is support in the UK by most ISP's - if you have an ethernet WAN
    interfaced router that you want to connect to a DSL Modem so that it
    gets the Public IP address you can configure the DSL Modem for
    rfc1483 bridging and then configure your Router to use PPPoE (and
    enter your DSL login details).
    Moonshine, May 21, 2006
  4. Simon Dean

    Simon Dean Guest

    Hypothetically, assume ISP's only support PPPoA. Would Ethernet modems
    still work in either the "RFC1483 Bridged" mode, or "Bridged Mode Only"?

    I mean people still come out with stuff like "So as it supports RFC1483
    (Bridge mode), you need to set it up in that mode to get it to work. In
    bridge mode the modem acts as a protocol converter converting ethernet
    into ADSL, this means that the host behind it has to handle all of the
    login and IP stuff. "

    Converts to ADSL? But is it PPPoA, or PPPoE? After the modem (if UK
    ISP's did not support PPPoE), would it really matter? Is it PPPoE
    because of the router?

    Then there's things like "As far as Smoothwal [my router] is concerned,
    there is no difference between PPPoA and PPPoE. It just cares about the
    "PPP" (authentication) part, as the "oA" (over ATM) or "oE" (over
    Ethernet) is an issue that needs to be handled by the modem/bridge to
    which Smoothwall's RED interface is connected"

    But then when you setup the modem in Bridged mode, you cannot specify
    PPPoA, or PPPoE. So the choice of modes, oA or oE must surely be handled
    by the router?

    And, what's the difference between RFC1483 Bridged, and "Bridged Mode Only".

    Simon Dean, May 21, 2006
  5. Simon Dean

    Moonshine Guest

    The best thing you can do is take a look at a diagram that shows the
    various ADSL protocol stack implementations.


    Hopefully that way you can see where the various bits fit.

    Bottom line is that in the UK at the user end PPP must be involved
    somewhere for IP address allocation and end user authentication.

    If you have 2 devices the functions can be split amongst them so long
    as the right stack layering is used.
    Moonshine, May 21, 2006
  6. Simon Dean

    Bob Guest

    I don't think that any UK ISP uses RFC1483 on ADSL, you want the other
    Bob, May 21, 2006
  7. Simon Dean

    Simon Dean Guest

    Yup. I get all that.

    But in this instance, being in bridged mode, the choice of oA or oE is
    decided by the device that does the authentication, in this case, the
    router. The PPPoE that's setup in the router, won't get magically
    changed by the modem to PPPoA, and it doesn't matter whether RFC1483 or
    "Bridged Mode Only" do, they both just pass "PPPoE" across.

    Bottom line, Is it physically impossible to have a bridged modem and a
    PPPoA connection when the router only does PPPoA??? So if ISP's did not
    support PPPoE, I would be stuffed unless I got an integrated router with
    PPPoA, or some other router that could do PPPoA through the modem via
    Simon Dean, May 21, 2006
  8. Simon Dean

    Simon Dean Guest

    That's what ADSLGuide suggest too. And all indications, as far as I can
    tell, suggest that when the modem is put into Bridged Mode,
    authentication passes to the router, or in my case smoothwall. Therefore
    in my mind, my smoothwall establishes a PPPoE connection. So ergo (in my
    mind) physically impossible to obtain a PPPoA connection when the modem
    is bridged.

    But that's what people suggest.

    Which then begs the question, if ISP's didn't support PPPoE, I couldn't
    have this setup, because it's physically impossible to do PPPoA (for
    this setup)

    ADSLGuide also say RFC1483 is irrelevent in the UK. Yet large numbers of
    people suggest it. I also don't know what it is, or how it differs to
    Bridged Mode Only, of which, there are no docs I can find.

    Simon Dean, May 21, 2006
  9. Simon Dean

    Bob Guest

    It's really better to think of it as PPPoEoA
    IIRC RFC1483 is just an extra header that's slapped-on the front of the
    AAL5 frame and doesn't do much except say: "this is ethernet", which isn't
    much use unless there is the possibility it might be something else.

    When I was recently looking for a modem, I looked into PPPoE and decided
    against it. I just found so many problem reports about it. Some people say
    that it wouldn't work until the ISP turned it on for them, someone said
    Tiscali's support told him they didn't support it at all - but he still
    got it to work. Lots of people complained that it's difficult to get
    technical support on it, even from good ISPs, the frontline people often
    just deny all knowledge of it. There is also the problem that it's only
    widely supported because it's a BT feature, if you move to LLU, or you ISP
    migrates to LLU, it may fail to work. There are also hassles with PPPoE
    and mtu sizes.

    I'm using FreeBSD so I went for an XModem, an Ethernet modem which
    terminates PPPoA itself and hands over the public address by DHCP. For
    Linux I'd probably go with an internal PCI modem. I think these are both
    better solutions.
    Bob, May 21, 2006
  10. Simon Dean

    Simon Dean Guest

    PPPoE, over the ATM network?
    That explains why it's not much use in the UK then I guess.
    Bringing me back to one of the original questions... with the modem in
    bridged mode, it functions just like a modem, so I would need my Linux
    box to do PPPoA authentication. Of which Smoothwall doesn't.

    I personally found setting it up rather easy. It's just trying to figure
    which bit of it decides on PPPoE or PPPoA and what the different
    bridging modes are. As I say, there's some people that say that PPPoE or
    PPPoA is decided and handled in the modem, which sounds like balls
    because once you set the bridging mode, you don't specify whether you
    wana oA or oE so I assert that's handled on the router itself. Why do
    things have to be so difficult. then they say "it really doesn't matter
    as long as the PPP bit is done". Then why do we have different modes?
    Ahh. That would be an infamous half bridge solution of sorts? You got a
    link for the XModem? the less things I stick inside the box the better.
    I don't trust things like the USB support, and PCI on Linux. sounds
    like it's asking for trouble trying to find the right devices that work.
    But it's always a possibility.

    Simon Dean, May 21, 2006
  11. Yet I believe that almost everywhere else in the world ADSL is
    provided using PPPoE not PPPoA. So there should be lots of support etc
    available on the web.
    Graham Murray, May 21, 2006
  12. Simon Dean

    Bob Guest

    You have to run PPP over something. In the US they use mostly ethernet, in
    Europe it's mostly ATM. BT added a hybrid version where ppp runs over
    ethernet, which in turn runs over their ATM network.
    adslnation.co.uk , but there are other similar products

    Bear in mind that the majority of ADSL routers are based on Linux.
    Bob, May 22, 2006
  13. Simon Dean

    Andy Furniss Guest

    I think the modem would do the pppoa. I don't see how you can bridge
    pppoa over ethernet because it doesn't fit into 1500 payload - that's
    why pppoe exists and you still have to lower the mtu.

    When using pppoe the modem will still have to make aal5 frames to turn
    to atm cells

    Andy Furniss, May 22, 2006
  14. Simon Dean

    Bob Guest

    When, I said yes I'd misread it as PPPoE.
    That doesn't make any sense, AALs run over cells. When using PPPoE the
    modem is bridging ethernet packets (carrying PPPoE) over ATM.
    Bob, May 23, 2006
  15. Simon Dean

    Bob Guest

    That "yes" was to authentication, rather than PPPoA.
    That doesn't make any sense, AALs run over cells. When using PPPoE the
    modem is bridging ethernet packets over ATM.
    Bob, May 23, 2006
  16. Simon Dean

    Andy Furniss Guest

    I was just trying to say that the modem still does much the same for
    pppoA and E - though with E you do get to run the code that does the

    AIUI both A and E will need padding out to make AAL5 frames before being
    turned into cells the A on pppoA is aal5. As you said PPPoEoA.

    It's the modem code that does this - even with PCI/USB it's not kernel
    code - you do get to see it on Bewan/Connexant. Both have tainting
    binary bits aswell.

    Andy Furniss, May 23, 2006
  17. Simon Dean

    Simon Dean Guest

    So... what happens if BT did not support PPPoE? Some sources say it
    makes no difference, some say this setup works (PPPoE on the computer,
    Bridged Ethernet Modem) only for the grace of BT and this therefore
    might not work on LLU.
    Simon Dean, May 23, 2006
  18. Simon Dean

    Simon Dean Guest

    Ok, so the modem will encapsulate whatever it has in AAL5 ATM. So, this
    will lead to PPPoAoA or PPPoEoA?

    I still don't see how the modem will do PPPoA when set to bridging mode,
    as choice of authentication type isn't available. So it must therefore
    be the responsibility of the router/computer that doesthe authentication.

    So I can see in that situation, the modem will do PPPoEoA where the
    router/computer only supports PPPoE.

    But I was under the impression from the likes of ADSLGuide, the only
    reason why this works, is because of the grace of BT who support a
    single PPPoE connection. If BT didn't support this, that setup would not
    work, would it? We'd either have to rely on PPPoA capable USB modems or

    So yes, if BT did not support PPPoE, then I presume that this PPPoEoA
    would not work. And ultimately, it has nothing to do with modem (unless
    it does the PPP authentication)?

    Simon Dean, May 23, 2006
  19. Simon Dean

    Dominic Guest

    My ADSL Ethernet modem (DSL-300G+) gives options... (which
    agree with the picture above!)

    PPPoA with VC Mux or LLC encapsulation, and username and
    password with chap authentication.
    PPPoE (no encapsulation options), and username and password
    with chap authentication.
    RFC 1483 bridge (no other options at all)
    IPoA (with LLC encapsulation), and IP address, Netmask,
    Gateway, and DNS entries.

    It's set to PPPoA and VCMux. It does all the
    authentication and login stuff, and
    gives the ethernet port it's plugged in to (in a linux box)
    the public IP address. (As
    I recall, it behaves exactly the same when set to PPPoE,
    but I may be wrong on

    Traceroutes from the linux box have the first hop as
    demon's home gateway... the
    modem doesn't count as a hop (although it does have a
    telnet / http interface on

    Dominic, May 23, 2006
  20. Simon Dean

    Andy Furniss Guest

    They didn't always and I am not sure if all ISP servers do (or need to).
    I think it won't work without BT - so you would need to let the
    modem/router use pppoa and give it your details. The modem/router you
    link to seems strange that it won't give you a real IP and not do nat -
    most will AIUI.

    I suppose it's bridged pppoe not bridged ethernet as such.

    Andy Furniss, May 23, 2006
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