In a failover configuration with Cisco Secure Firewall Threat Defense (FTD) devices, ensuring that users on the internal network can continue to communicate with outside devices if the primary device (FTD1) fails requires the implementation of a stateful failover link. The stateful failover link allows the secondary device (FTD2) to maintain session information and state data, ensuring seamless failover and minimizing disruptions.
Steps to implement a stateful failover link:
* Physically connect a stateful failover link between FTD1 and FTD2.
* Configure the stateful failover link in the FMC.
* Ensure that both devices are properly synchronized and that stateful failover is enabled.
* Deploy the changes to both FTD devices.
By configuring a stateful link, the secondary FTD can take over active sessions without requiring users to re-establish their connections, thus ensuring continuous communication.
References: Cisco Secure Firewall Threat Defense Configuration Guide, Chapter on Failover Configuration.

Explanation
https://www.cisco.com/c/en/us/support/docs/security/firepower-management-center/212699-configure-ftd-high-aConditions In order to create an HA between 2 FTD devices, these conditions must be met:
Same model
Same version (this applies to FXOS and to FTD - (major (first number), minor (second number), and maintenance (third number) must be equal)) Same number of interfaces Same type of interfaces Both devices as part of same group/domain in FMC Have identical Network Time Protocol (NTP) configuration Be fully deployed on the FMC without uncommitted changes Be in the same firewall mode: routed or transparent.
Note that this must be checked on both FTD devices and FMC GUI since there have been cases where the FTDs had the same mode, but FMC does not reflect this.
Does not have DHCP/Point-to-Point Protocol over Ethernet (PPPoE) configured in any of the interface Different hostname (Fully Qualified Domain Name (FQDN)) for both chassis. In order to check the chassis hostname navigate to FTD CLI and run this command

Traditionally, a firewall is a routed hop and acts as a default gateway for hosts that connect to one of its screened subnets. A transparent firewall, on the other hand, is a Layer 2 firewall that acts like a "bump in the wire," or a "stealth firewall," and is not seen as a router hop to connected devices. However, like any other firewall, access control between interfaces is controlled, and all of the usual firewall checks are in place. Layer 2 connectivity is achieved by using a "bridge group" where you group together the inside and outside interfaces for a network, and the ASA uses bridging techniques to pass traffic between the interfaces. Each bridge group includes a Bridge Virtual Interface (BVI) to which you assign an IP address on the network. You can have multiple bridge groups for multiple networks. In transparent mode, these bridge groups cannot communicate with each other. https://www.cisco.com/c/en/us/td/docs/security/asa/asa97/configuration/general/asa-97-general-config/intro-fw.html

Available under the Advanced tab of the access control policy, to enable or disable EVE.
https://www.cisco.com/c/en/us/td/docs/security/secure-firewall/management- center/snort/720/snort3-configuration-guide-v72/m_encrypted-visibility-engine.pdf