PPPoA and PPPoE Bridging

Unanswered Question
Jan 4th, 2008


I don't have that much experience with PPPoA protocol, but I was wondering if it is some way possible to do bridging on the DSL modem using PPPoA just as in PPPoE?

When using ADSL with PPPoE you can setup the modem to act as a bridge, it will bridge between the pstn and Ethernet. Therefore a Cisco router can use PPPoE encapsulation and authentication to connect. I would like to find a way to do a similar thing, where my router connected to the modem will have a public IP address and something like a dialer interface but without ATM interface present just Ethernet.


I have this problem too.
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Paolo Bevilacqua Fri, 01/04/2008 - 14:36

No, you cannot bridge PPPoA like PPPoE.

The reason why is that PPPoE has the ethernet header on which bridging is based, while PPPoA is a purely point-to-point protocol that you must terminated in the router, and cannot be carried on a LAN.

However you will find that most ISPs supporting PPPoA, do work with PPPoE also, due to their equipment supporting both transparently.

Hope this helps, please rate post if it does!

mohammadpatel Sat, 03/22/2008 - 06:43

There are many other modems/routers such a Dlink, 3com and others which offer pure bridge feature with PPPoA connection. If you use the bridge mode, the router will connect to the internet but instead of keeping the IP received from the ISP, it will pass it on to the device (PC) connected on the Ethernet interface of the router. I don't understand why Cisco routers doesn't support that.

Paolo Bevilacqua Sat, 03/22/2008 - 08:30

Are you sure you're talking about PPPoA and not PPPoE ? The former is not being very used anymore, beside if things were to work as you say, router should be "master" side pppoe, something complicated and probably unnecessary, considering Cisco outer has the best impletantion fo nat and everything works anyway.

mohammadpatel Sat, 03/22/2008 - 13:47

Yes - I am very sure it's PPPoA. The ISP here doesn't offer PPPoE and all the implementations are based on PPPoA. There are other routers like I said, works fine with this by doing the PPPoA bridge connection and the PC gets the IP address without making any dialup connection. At times you don't want to use NAT on the device for e.g. if you want to connect a PIX device with the ADSL and need the Public IP to be assiged on the WAN interface of the box then you are stuck? :)

Paolo Bevilacqua Sat, 03/22/2008 - 16:04

Please be aware that many BRAS do automatically support both PPPoA enad PPPoE transparently, the ISP may not even be aware of that.

So if you connect the cisco in PPPoE it might work and then you can bridge to the LAN.

If that doesn't work, configure static NAT with the internal address of the PIX and everything will work anyway.


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