Some background - https://forum.vyos.io/t/pppoe-server-dual-stack-ipv4-ipv6-buggy-soho-routers-that-refuse-ipv6/9010/4
traffic capture - http://22.214.171.124/phicomm/phicomm6.cap
RFC1661 - 3.7 Link Termination Phase - Implementation Note:
The closing of the link by LCP is sufficient. There is no need for each NCP to send a flurry of Terminate packets. Conversely, the fact that one NCP has Closed is not sufficient reason to cause the termination of the PPP link, even if that NCP was the only NCP currently in the Opened state.
Some cheap buggy unsupported SOHO routers (specifically, Phicomm KE2M) as PPPoE clients don't support IPv6, but try to negotiate it anyway and then fail, client sends IPV6CP terminate request, and VyOS PPPoE server responds by terminating LCP, even though it could still work fine as IPv4 only. This makes IPv6 deployment difficult, as changing ppp-options ipv6 "deny" to "allow" breaks existing IPv4 customers who happen to have such buggy routers.