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As far as I know the dsl modem tries to authenticate at the ISP via CHAP in a PPPoE-frame which is in an Ethernet-frame. But in a "normal" Ethernet frame needs a destination MAC address. How does my DSL-modem get the destination-address of the ISP-device on the other end?

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PPPoE clients use PPPoE Active Discovery to discover PPPoE Access Concentrators (servers) on the network. The first packet the client transmits is a PPPoE Active Discovery Initiation (PADI) which is sent to the Ethernet broadcast address (all ones in binary, all f's in hex: ff:ff:ff:ff:ff:ff). And PPPoE ACs on the network respond with PADO's (Offers). The client notes the source MAC address of the PPPoE AC that sent the PADO the client is interested in, and replies to that MAC address with a PADR (Request). If the AC accepts the request, it responds with a PADS (Session-confirmation).

PPPoE Active Discovery uses EtherType 0x8863.

After Active Discovery completes, the PPPoE client and AC start sending frames using the PPPoE Session EtherType 0x8864. These frames contain PPP messages, including LCP, authentication, and IPCP negotiation, and eventually IP datagrams in PPP.

So the CHAP frames are sent to the unicast MAC address of the PPPoE Access Concentrator, which was discovered via PPPoE Active Discovery.

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  • Do you have a official document link where that is specified?
    – kimliv
    Apr 9, 2014 at 14:18
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    @kimliv tools.ietf.org/html/rfc2516
    – Spiff
    Apr 9, 2014 at 15:53
  • In which of these PPPoE Active Discovey steps the IP is assigned?
    – Mohamad
    May 2, 2014 at 8:46
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    @Mohamad It's not assigned during PAD, it's assigned during IPCP negotiation (which is part of the PPP negotiation at the start of the PPP session).
    – Spiff
    May 2, 2014 at 10:12
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It sends to ff:ff:ff:ff::ff:ff which is the "broadcast" mac address. If you receive a frame with this destination don't drop it. It's for you!

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  • Not quite. The CHAP frame would go to the Access Concentrator's unicast MAC address, which was discovered during PPPoE Active Discovery.
    – Spiff
    Apr 9, 2014 at 11:01

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