PPPoE Overview and Troubleshooting ways

Using Vigor Router to create a PPPoE connection for accessing the Internet is a usual thing for us. This article demonstrates the 6 stages of the process that the Vigor Router create a PPPoE connection.
We could capture WAN packets to help us to figure the PPPoE connection issue that we may meet.

1st stage: PPPoED session creation

  1. Vigor Router acts as PPPoE Client that will send a PADI packet to initial the PPPoED session.

  2. PPP Server will respond PADO after receiving PADI

  3. After receiving PADO, the PPPoEClient which the Vigor Router will send PADR, then the server will respond to PADS.

  4. PPPoED session is created after receiving PADS packets.

The PPP server will provide PPPoE Session ID in PADS packets and the following stages such as LCP Configuration, Authentication, IPCP will all use this PPPoED session ID.

In this stage, we may meet a problem that Not receiving PADO from the PPP server, it could be the following reasons:

When the PPPoE client or server wants to disconnect the PPPoED session, they will send PADT (Terminate) with this PPPoED session ID.

2nd stage: LCP Configuration negotiation

Both PPPoE Client and Server will send LCP configuration request to each other for negotiating MRU, Magic number, and the Authentication protocol. Client and Server need to ack each other for moving to the next stage.

3rd stage: PAP or CHAP Authentication

PPPoE Client will send the PPPoE Username and Password for Server's verification, and the server will reply with an Ack or Nak packet as the authentication result.
If the server doesn't respond with Ack, it means the user name or password is incorrect. We could ask the customer to confirm the authentication information again.

4th stage: CCP Negotiation

Some servers or clients may send CCP packets for negotiating the compression protocol to encrypt the PPP header.
Vigor Router won't send CCP packets in creating a PPPoE connection.

5th stage: IPCP Negotiation

Both Client and Server will send IPCP configuration packets to negotiating the IP, DNS server IP and so on.
The process will be finished after both the client and server ACK each other request. And it's normal that they negotiate several times, then end up with the server response the IPCP ACK.

6th stage: LCP Echo Request/ LCP Echo Reply

After the PPPoE connection is established, both the client and server may send LCP Echo to ensure if the peer is alive or not. Vigor Router will send LCP Echo-Request when it detects the connection is idle. On the contrary, the router will not send LCP Echo-Request when it detects there are traffics passing through the connection.

If the PPP server doesn't respond to the LACP Echo 2 times, Vigor Router will send an LCP Terminate-Request to disconnect the PPPoE session.
If the PPP server doesn't respond to the LCP Terminate-Request 3 times, Vigor Router will send PADT to close the PPP connection.

what we can do for troubleshooting the WAN PPPoE connection problem?

  1. Check the physical link connection first.

  2. Capture the WAN packets to check the connection stops at which stage and following the article to troubleshooting

  3. If none of the above solve your issue of PPPoE connection. Please provide the following information to the Draytek Support for further investigation:

    • Model name and firmware version.

    • WAN Syslog and packets captured

Published On:2020-12-25 

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