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Case.1 We know that when router connected via point to point(PPP) protocol then ARP isn't used to know the destination Mac address.

Case.2 When two router connected via PPP as well as Ethernet which is PPPoE, then also ARP isn't used to know destination Mac address.

My question is for case2 PPPoE request frame is used to know the destination Mac address?

And in PPPoE request frame PPP is also used as well as in Ethernet frame PPPoE frame(which contained PPP frame) also used to know the destination Mac address?

And in Ethernet frame destination address should be ff:ff:ff:ff?

Like the below image:

enter image description here

2 Answers 2

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You're getting too hung up on the ethernet side of the equation. Yes, ethernet is a multi-point medium. PPPoE does not use it that way. The PPPoE discovery/setup process creates a single, one-to-one path across the multi-point ethernet medium. Typically, there's only one server, so all clients learn the MAC from it's broadcasts; the server then learns the various individual clients when they transmit. If there are multiple servers, a client can associate with any of them, including all of them. (if there are multiple ISPs on a shared link, you'll only be associating with your ISP(s))

Keep in mind, PPPoE may only be ethernet for a few feet -- between the customer's router and ISP's modem. I've seen it terminate on the DSLAM, as well as transported across the country (USA) via tunnels to a single, central server. (i.e. wholesale DSL)

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  • Pppoe with Ehthnet you said multiple devices are possible through switch. But only PPP more than one relationships are possible? Or with PPP only two devices connection are possible by one link?
    – Alok Maity
    Mar 17 at 17:04
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    A PPPoE session is a single, one-to-one connection. There can be more than one session on a link -- that's why the protocol has a session id. Just like TCP is a single, one-to-one connection, where there are more one connection per link.
    – Ricky
    Mar 17 at 17:09
  • That's understand. But I am asking without using Pppoe, if I using only PPP then more than one session on a link is possible?
    – Alok Maity
    Mar 17 at 17:12
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    You can't use PPP over Ethernet without the addition of the PPPoE protocol bits ("PPPoE packet encapsulation format") because ethernet is not a point-to-point medium - there aren't just two nodes on an ethernet network. (even when their might physically only be two, the medium is designed for more. MACs must be used to identify nodes.)
    – Ricky
    Mar 17 at 17:20
  • Ethernet is not a point-to-point but a multipoint technology (even when two devices are connected back to back). Ethernet is designed to allow multiple devices to share a common medium, called a “broadcast domain”.
    – Alok Maity
    Mar 21 at 12:17
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PPPoE is defined in RFC 2516 and includes a discovery stage for locating PPPoE servers/access concentrators:

5. Discovery Stage

There are four steps to the Discovery stage. When it completes, both peers know the PPPoE SESSION_ID and the peer's Ethernet address, which together define the PPPoE session uniquely. The steps consist of the Host broadcasting an Initiation packet, one or more Access Concentrators sending Offer packets, the Host sending a unicast Session Request packet and the selected Access Concentrator sending a Confirmation packet. When the Host receives the Confirmation packet, it may proceed to the PPP Session Stage. When the Access Concentrator sends the Confirmation packet, it may proceed to the PPP Session Stage.

All Discovery Ethernet frames have the ETHER_TYPE field set to the value 0x8863.

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  • "one or more Access Concentrators sending Offer packets "--- why more Access Concentrators are present? There is point to point relationship, how more than one Access Concentrators are present? 2 hosts are present between one link?
    – Alok Maity
    Mar 17 at 15:01
  • Multiple access concentrators are possible - e.g. in a shared local loop (xDSL) with multiple ISPs offering dial-up services. A point-to-point relationship is established with each AC - usually only a single one but multiple simultaneous relationships are possible as well.
    – Zac67
    Mar 17 at 15:21
  • But Layer3 with only PPP (not using ppoe)there exists more than one relationships?
    – Alok Maity
    Mar 17 at 16:10
  • Pppoe with Ehthnet you said multiple devices are possible through switch. But only PPP more than one relationships are possible? Or with PPP only two devices connection are possible by one link?
    – Alok Maity
    Mar 17 at 16:28
  • No, each PPP(oE) session is always point-to-point - between two hosts/devices. I wanted to point out that there can be sessions with multiple partners potentially.
    – Zac67
    Mar 17 at 17:29

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