as one who has been burned when topologies are not congruent, i gotta ask. if i do not anticipate v4 and v6 having different topologies, and all my devices are dual-capable, would you still recommend mt for other than future-proofing?
Personally, if my v4 and v6 topologies are not different, I'd run ISIS and not run MT. MT for me is to make v4 and v6 have different control planes (even though it's using the same protocol), thus I see little difference in running OSPFv3+ISIS, or running ISIS-MT for v4+v6.
I argue that it's better to have different control planes for v4 and v6 and make it obvious (OSPv3 / ISIS), than to use ISIS-MT and "obfuscate".
the real control plane is bgp. is-is is for recursive resolution to find bgp's next hop interface, fertig. so the simpler the better. i am annoyed enough that bgp4 and bgp6 peerings and configs are overly divergent. running a different igp for 6 and 4 would not make me happy. randy