Hi,
right from the start I admit I am completely new to RouterOS/MikroTik and to IPv6 so I ask for a little bit of patience first.
Right now the question is theoretical but it can hit me every time. I hope it does.

I need to interconnect two branches. On both sides I have an MikroTik/RouterOS 6.15 router. The ISPs on both sides are providing IPv6. On one of the branches IPv6 on MikroTik is already up and working. I know hot to configure this. IPv6 ping to Google and name resolution over IPv6 works.

Both branches of course use IPv4 internally and this should not be changed. For security reasons both routers should use IPSec for the connection.

How do I send the IPv4 traffic over the IPv6 IPSec tunnel?

Best regards.

Just set up ipsec peering between the your hosts and set up some EoIPv6 tunnel that will be encrypted by ipsec. And EoIPv6 tunnel will pass any traffic that ethernet interface can pass.

Thank you for your answer.
That would also pass dhcp requests and all other arp stuff right?

ok, perfect. I have done an EoIPV6 and it works fine. This tunnel pass even the MAC address so we have a "cable" between two IPV4 branch through an IPV6 network.
How is it posible to cipher the tunnel??