Having worked for seveal SP's 'tier 1' and otherwise along with a couple of router vendors IMO MT is one of those thing people ask for just in case. Sure we _could_ always find a use for it, but we don't always look at the potential diffrent IGP topologies are going to cause for our NOC staff @ 2am over a holiday weekend when some does decide to break.
Thanks all of you to provide your inputs on my questions!
The main idea behind Multitopology in IS-IS is to enabling the IPv6 routing
in the redundant part of the network so that way I will not mess around with
the current IPv4 routing or services which is running or serving to
customers currently! so by migrating redundant part of the topology to IPv6
using Multitopology IS-IS and make it that part as a active for IPv6 for
testing how it works! and then I can enable the IPv6 on my whole network! I
guess that might be the good benefit.
Same thing we can do with OSPFv3 also as I can enable IPv6 routing using
OSPFv3 on my redundant part of the network and after successful migration i
can enable it on my whole network!
But again as far as expansion is concern IS-IS is good protocol to consider.
OSPF does have bit more complexity in terms of operation. again the one
question is how about the router resource utilization for both the protocol
if I will be running IPv6 and IPv4 in the network!
One more question: do we need to enable the IPv6 on each and every router of
the service provider network including P routers also? does it really
required to run IPv6 on each and every router? or running it on only PE
router is sufficient to support the customers needs?