Comcast’s native IPv6 deployment by default sends DHCPv6 prefix delegations of /64 (which is too small!), but their routers are configured to issue up to a /60 when the larger prefix is specified in the DHCP request.
The way this is implemented is outlined in RFC 3633, here. The relevant bit:
A requesting router may set the IPv6 prefix field to zero and a given value in the prefix-length field to indicate a preference for the size of the prefix to be delegated.
It would be great if we could get a setting within the RouterOS DHCPv6 client to set the prefix-length field in outgoing requests, to provide the ability to get the larger prefix from ISPs who implement prefix delegation in this way.
Oh, nice! I had checked the release notes but it doesn’t look like it’s in there - that’s the kind of sneaky feature add that I absolutely approve of. Thanks for the info!