This article relates to Cisco TelePresence Video Communication Server Expressway.
Q. Do all the VCSs in a cluster need to be in the same subnet?
A. No, peers in a VCS cluster can be in separate subnets. VCS peers communicate with each other using H.323 and SSH messaging, which can be transmitted across subnet boundaries.
Note that TANDBERG Gatekeepers and 3rd party alternate gatekeepers behave differently and do have to reside in the same subnet.