here is my question
If we have 3 CUCM clusters
o Cluster A - version 8.5
o cluster B - version 11.5
o cluster C - version 9.1
We setup an intercluster trunk between them.
Is there any version requirement to build these ICT?
Thanks in advance
No, there is no version requirement. If you are building SIP intercluster trunks (recommended!), there are some behavioral differences for the SIP protocol with regards to MTP and FQDN on the 8.5 cluster. If you build H323 intercluster trunks they all behave the same way between those versions as far as I know.
However, if you are planning to deploy Jabber, URIs, ILS and other newer technologies they all require SIP trunks so I encourage you to go that route with your intercluster trunks and not H323. If you go with H323, at some point you will likely have to configure SIP trunks later anyway.
SRND recommends SIP trunks for 8.5+ and only use ICTs to older releases.