When reconfiguring an existing Cisco FTD from transparent mode to routed mode, an additional action that must be taken to maintain communication between the two network segments is toupdate the IP addressing so that each segment is a unique IP subnet. This is because in routed mode, the FTD device acts as a router hop in the network and requires each interface to be on a different subnet. In transparent mode, the FTD device acts as a layer 2 firewall and does not require different subnets for each interface1.

The other options are incorrect because:

Configuring a NAT rule so that traffic between the segments is exempt from NAT is not necessary to maintain communication between the two network segments. NAT is used to translate IP addresses between different networks, but it does not affect the routing of packets. Moreover, NAT is optional in routed mode and can be disabled if not needed2.

Deploying inbound ACLs on each interface to allow traffic between the segments is not required to maintain communication between the two network segments. ACLs are used to control access to network resources based on source and destination addresses, protocols, and ports. They do not affect the routing of packets. Furthermore, ACLs are optional in routed mode and can be configured as needed3.

Assigning a unique VLAN ID for the interface in each segment is not relevant to maintain communication between the two network segments. VLANs are used to create logical groups of hosts that share the same broadcast domain, regardless of their physical location or connection. They do not affect the routing of packets. Besides, VLANs are not supported in routed mode and can only be used in transparent mode4.