A bootable ISO file is a complete installation package that allows you to install or upgrade your CUCM server from scratch. This file contains all the necessary files and instructions to boot the server and initiate the installation process. Having a bootable ISO file is essential for several reasons:
CUCM 12.5 dropped support for legacy M3 series UCS servers. Attempting to boot the ISO on unsupported hardware will show a red fatal error. Check the for 12.5 before downloading. cucm 12.5 bootable iso download