More details to follow - I have set up a problematic router (Phicomm KE 2M - manfacturer no longer exists) as PPPoE client on a dedicated VLAN, and VyOS 1.3.1 as PPPoE server with IPv6 enabled (ppp-options ipv4 require, ipv6 allow), and captured the traffic. I see normal PPPoE discovery, LCP negotiation, PAP authentication, IPCP negotiation- until now, as expected. Then, the router (which in its web GUI has no traces of any IPv6 support!) starts with IPV6CP (client sends random interface identifier to server). Then IPCP and IPV6CP proceed in parallel, IPv4 is negotiated, and soon after, the client sends IPV6CP termination request (the packet contains the string “Interface configuration failedace”), then the server sends LCP termination request and finally PADT. RADIUS logs show Start with all the IP addresses, and then Stop with “Acct-Terminate-Cause = User-Request” in the same second. All of this repeats every 33 seconds, traffic capture is available here - http://91.224.224.43/phicomm/phicomm6.cap . Other routers either support IPv6 and work dual stack, or don’t support IPv6 and work as IPv4 only, with the same PPPoE server configuration. Unfortunately, these routers were cheap and a significant number of customers has them. My conspiracy theory is that they could no longer be sold in China as the ISPs there already support IPv6, so they sold them to Poland where IPv6 support was not universal yet so the breakage was not apparent (IPv4 only works fine). But, it is the PPPoE server that terminates LCP in response to IPV6CP termination by the client, perhaps the server could just accept IPv6 termination but instead continue to work as IPv4 only?