An organization uses Cisco FMC to centrally manage multiple Cisco FTD devices The default management...

 The management port on Cisco FMC is used to establish a secure connection with the managed Cisco FTD devices. If the default management port (8305) conflicts with other communications on the network, it must be changed on both the Cisco FMC and the Cisco FTD devices. This cannot be done automatically by the Cisco FMC, as it would lose connectivity with the devices. Therefore, the administrator must manually change the management port on the Cisco FMC and all the managed Cisco FTD devices using the command line interface (CLI). The steps to change the management port are as follows:

Log into the CLI of the Cisco FMC and the Cisco FTD devices using a console connection or SSH.

Enter the configure network {ipv4 | ipv6} manual ip_address netmask data-interfaces command to change the management port on the Cisco FMC. For example, configure network ipv4 manual 10.10.10.10 255.255.255.0 data-interfaces changes the management port to 10.10.10.10/24.

Enter the configure network {ipv4 | ipv6} manual ip_address netmask gateway management-only command to change the management port on the Cisco FTD devices. For example, configure network ipv4 manual 10.10.10.11 255.255.255.0 10.10.10.10 management-only changes the management port to 10.10.10.11/24 and sets the gateway to the Cisco FMC’s management port.

Save the configuration and restart the Cisco FMC and the Cisco FTD devices.

Verify the connectivity between the Cisco FMC and the Cisco FTD devices using the show managers command on the Cisco FTD devices and the show devices command on the Cisco FMC.

References :=

Firepower Management Center Device Configuration Guide, 7.1 - Device Management

Change management port fmc 1600 - Cisco Community

Solved: FMC 2120 FTD Management Only Port - Cisco Community

Change the FMC Access Interface from Management to Data