The correct answer is to enable two-factor authentication through a RADIUS server, and then join the cluster via the SEG CLI. Two-factor authentication is a security feature that requires users to provide two forms of identification before accessing the SEG, such as a username and password, and a one-time code or token. This adds an extra layer of protection against unauthorized access and phishing attacks. The SEG supports two-factor authentication through external RADIUS servers, which can be configured on the System Administration > Users page in the web interface, or the userconfig command in the CLI. See User Guide for AsyncOS 14.0 for Cisco Secure Email Gateway - GD (General Deployment) (Section: Two-Factor Authentication) for more details.
To join a cluster, the SEG must communicate with other cluster members using either SSH or CCS (Cluster Communication Service). The cluster communication port and method can be configured on the Network > Cluster Communication page in the web interface, or the clusterconfig command in the CLI. See User Guide for AsyncOS 14.0 for Cisco Secure Email Gateway - GD (General Deployment) (Section: Cluster Communication) for more details.
If two-factor authentication is enabled on the SEG, it cannot join a cluster using the web interface, because the web interface does not support two-factor authentication for cluster operations. Therefore, the SEG must join the cluster using the CLI, and provide a pre-shared key that matches the cluster's admin passphrase. The pre-shared key can be configured using the clusterconfig > prepjoin command in the CLI. See User Guide for AsyncOS 14.0 for Cisco Secure Email Gateway - GD (General Deployment) (Section: Creating and Joining a Cluster) for more details.
The other options are incorrect because they either use the wrong authentication server (TACACS+ instead of RADIUS), or the wrong communication method (GUI instead of CLI). References:
* User Guide for AsyncOS 14.0 for Cisco Secure Email Gateway - GD (General Deployment)
* Configure an Email Security Appliance (ESA) Cluster - Cisco
Reference:
https://www.cisco.com/c/en/us/td/docs/security/esa/esa11-0/user_guide_fs/b_ESA_Admin_Guide_11_0/b_ESA_