Why configure modem as bridge vs PPPoE?

Discussion in 'General Discussion' started by jlentz, Aug 24, 2006.

  1. jlentz

    jlentz LI Guru Member


    I've seen where people have changed their dsl modem from performing the PPPoE connection to just a bridge modem - and let their router do the PPPoE login. Is there an advantage to this? I've currently got my modem set for the default PPPoE and the router just plugs into it. I could switch the modem to bridge mode (as I've read that others have done), but I'm not sure why I'd want to do that.


  2. Esquire

    Esquire Mesquire Staff Member Member

    At least when it comes to troubleshooting, there is one less equipment to check.

    My DSL modem has always been locked by my ISP as a bridge modem.
  3. jlentz

    jlentz LI Guru Member

    Does having the modem in bridge mode rule it out completely as a problem? Or can it still be a culprit in bridge, but without the ability to IP to it?

  4. Esquire

    Esquire Mesquire Staff Member Member

    Good point! Yes, as with any hardware it can still be the culprit - if it fails, it fails.

    For me, it's always one less equipment to configure since my ISP locks them modems this way - I just plug it in, power up and wait for the signal to synchronize.
  5. d__l

    d__l Network Guru Member

    With the AT&T Speedstream 4100 modems, it is generally a better idea to allow the modem to make the PPPoE connection. The modem then can assign a fixed private IP by DHCP to the router. With this set up, the modem's GUI can be accessed at any time to check DSL stats (even while the PPPoE connection is up).

    If the modem is bridged, the PPPoE connection must be taken down and some router settings must be changed to access the line stats.

    Actually this modem has three PPP modes: normal PPPoE from the modem, a PPPoE pass through which might be thought of as a semi-bridge mode, and the true pure bridge mode.
  6. jlentz

    jlentz LI Guru Member

    The Speedstream 4100 is the type I have. I've been experiencing some intermittent connection problems and I'm trying to see if I can do anything to reduce it. I think it is on the ISP side, since when it goes bad I get the blinking internet light and the modem diagnostics show their side as bad. Unfortunately, it doesn't fail long enough for them to diagnose and fix the problem permanently. I was hoping that switching to bridge mode would make it more reliable.
  7. d__l

    d__l Network Guru Member

    So you have you modem making the PPPoE connection and issuing the IP,, to your router so you can access the modem's GUI? Then what do the modem's tests under Check Connection button show right when you are having your connection problems?

    There is an undocumented feature of the AT&T 4100 that it will act as a PPPoE pass through (the semi-bridge mode) if the UserID/Password are left blank and the attached router makes the PPPoE connection. This is similar to the "PPP is on the computer" mode. A press of the reset button is the fastest way to clear your UserID/Password (be certain that you know your password before erasing it :) ). You might try this mode and let your router make the connection to see if that is more stable.

    The "Bridged Mode (PPPoE is not used)" is primarily for true static or DHCP-type DSL connections.
  8. jlentz

    jlentz LI Guru Member

    When I get the failure, it is in the PPPoE tests that it fails. I've managed to get hold of SBC when it is in the failed state and they say "Yep, it is failed". It doesn't stay failed long enough for proper troubleshooting. I'll have to try the semi-bridge mode and see if it helps. Thanks.
  9. ka9yhd

    ka9yhd LI Guru Member

    The reason that I have always let the router do the PPPoE connection is that my modem will not do PPPoE. Weather it has been a spare pc running a Linux router/ firewall app or the recently aquired WRT54GL, I have had the router make the PPPoE connection.

    I have heard of a few people using a ethernet switch after their modem and letting each PC do the PPPoE. But then there is no firewall before the PC's.
  10. d__l

    d__l Network Guru Member

    What router model will you be using to make the PPPoE connection?

    I generally find that Keep Alive setting at the default 30 secs works fine for me. I've actually been able to increase that time to 90 secs with no stability problems. The increased time cuts some of the unnecessary traffic that the router is continually sending out the WAN.

    I haven't experimented with longer intervals yet. I chose 90 secs because I know that AT&T's IP leases are a litttle longer than that. Something on the order of at least 2 minutes. I've been officially told that the leases are supposed to be 5 minutes, but that just isn't so.
  11. jlentz

    jlentz LI Guru Member

    I currently have a DI-624 as the AP, and I'm working on getting a WRT54G as a Wireless Eth bridge. However, tomorrow I plan on getting another WRT54G to use as the AP, since I like the firmware (HyperWRT) much better in the WRT than the stock dlink stuff.
  12. ka9yhd

    ka9yhd LI Guru Member

    I have the WRT54GL set to keep alive 30 seconds. Seems to work fine with Covad
  13. d__l

    d__l Network Guru Member

    Is that 4100 fairly new? A few others have reported some connection drop problems with the 4100. I think there problems were when the modem was making the PPPoE connection. There may be a batch of modems that went out in the past few months that had higher than normal problem rates.

    The log in the modem may give you some clue as to why it is dropping the PPPoE. If the router holds the PPPoE connection better, you may be able to get a replacement under warranty if it is less than one year old. You might have to have some lines from the modem log to prove that it is causing the PPPoE drop for the warranty exchange.

    There is another way to set up the modem and router if you have a spare 5-port switch and an old router that allows the router to make the PPPPoE connection and be able to access the modem GUI for DSL stats at the same time.
  14. YeOldeStonecat

    YeOldeStonecat Network Guru Member

    I always flip them to pure bridged mode...I don't like double NAT'ing. Hinders performance, makes opening/forwarding ports for public services more difficult..many apps act flakey through double NAT. They're often not the highest performing routers either....so I let them just do their bridging.
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