I can’t find it anymore, but on the old forum there were a few threads about IPv4+v6 Dual Stack and how they didn’t play nicely with mwan3 Load Balancing & Failover, especially when only ONE wan interface has a V6 assigned. The discussion mentioned that changes were scheduled to come in FIrmware 7.5, which has now been released. But, sadly it doesn’t seem that anything changed.
I found a couple of related pages about mwan3 and how it requires various hacks to make it work with IPv6. So basically the advice seems to still be “disable IPv6 if you are using multiwan / failover / load balancing”. I am posting here to confirm if this is the “correct” assumption.
The main problem we face, is that if IPv6 is enabled, but e.g. only exists on the MOBI interface(s), you can get a huge data bill as many devices now prefer to use IPv6, and they will happily route all traffic over your slow/expensive LTE interface when the fast/free Fiber WAN sits unused.
This case is not considered a high priority one, so we decided to postpone the fix to a future firmware update. But I want to make sure you understand that this fix will not solve the problem you reported, because the router does not decide whether to use IPv4 or IPv6. That depends on the applications that run on your devices.
The router simply returns IPv4+IPv6 addresses via DNS queries, and then the client chooses which version to use. Most applications prefer IPv6, and thus will use the mobile connection.
If you are using IPv6 to reach the router itself, then it should be enough to disable RA and DHCPv6 on the LAN interface DHCP settings. This way the router will still obtain an IPv6 address, but it simply won’t distribute it to the LAN devices.
