I have a router in one provider who didn't read RFC 6177 and thus assigns my MikroTik router 1 (YES, I said ONE) IPv6 in ether1, using DHCPv6. It also tells me gently to set up a default router to this interface. They also block protocol 41, because they don't want my life to be too easy.
To be able to deploy some IPv6 there I set up a 6to4 tunnel with IPsec to another of our offices (fortulnately they don't block IPsec). I called it sit1 and set up the proper routes to that a /52 of one of my /48 from HE is assigned there.
Now, the problem is: Everything works well if I don't accept their "native" IPv6 default route. All the traffic goes through sit1, using the address I set up in bridge.
Now, if I add their native default IPv6 route, as the distance is smaller than to the tunnel, all traffic flows through ether1 where it is dumped into the bit bucket.
This is a general problem in the face of dual IPv6 providers... how is typically approached in RouterOS the problem of forwarding packets with src AAAA:AAAA::/N through an interface that routes only BBBB:BBBB/M?
Regards