I'm planing to upgrade CUCM to 9.1(2)SU3, I need to know recommended version IOS for VGW and recommended IM & P version.
If anyone can share a guide this will be a great help
IM and Presence Service Release 9.1(1a) is compatible with Unified Communications Manager Release 9.1(2).
For IOS< you can take the IOS versions listed under cucm version 10.0(1) as the minimum recommended version from the following link since 9.1 is not explicitly mentioned
Always attempt to match exact version of CUCM to IMP, if one is not available then make sure major versions match i.e. 9.1.
As to GW support, unless you are using MGCP the version is irrelevant and you can use any. You did not specify your router model, but assuming it's 29/39XX I would recommend latest 15.4M train.