How does the authentication take place in a pppoe bridge setup assuming I have a bridge modem and a router that is capable of maintaining a bridge connection both connected to an un-managed switch?

As far as I can tell the PPP connection is encapsulated by the modem as ethernet frames and then the router interprets those frames for authentication purposes. What I don't understand is the communication between the router and modem. Does the modem have an address on the switch and the gateway of the router is that of the modem? Is authentication information included in every packet that passes through the modem?

1 Answer 1


In this case, your router talks PPP to the provider directly.

But first, the router puts this data into Ethernet frames which it transmits to the modem. (hence PPP-over-Ethernet)

Since the modem is in bridge-mode, it won't interpret the frames, only encapsulate them itself in AAL (ATM Adaption Layer) or whatever in order to transmit over the public network to the provider.

The provider then authenticates you etc.pp.

Other cases:

  • "half-bridging": the modem performs the PPP authentication for you but doesn't do natting or anything but forwards the Ethernet traffic to an internal host/router which is supposed to do the rest
  • Finally, the modem can also assume the role of a router, in which it does the PPP, NATting/routing, etc.
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    How does the modem know which packets are for it when authenticating for the first time? Does the router send out a broadcast packet asking for pppoe ability or is there something more complex going on? Does the router address the modem as an ip address address the ppp provider as a mac address? Commented Jul 29, 2014 at 19:29
  • 1
    Look up PPPoE Discovery client to server initiation (PADI). All is explained there. Since your modem is in bridge mode (bridge being another term for switch) and PADI packets being Ethernet broadcasts, they reach the provider just fine.
    – Marki
    Commented Jul 29, 2014 at 19:41

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