Which of the following is an authentication method that requires an external database to be implemented with Cisco ISE?

The authentication method that requires an external database when implemented with Cisco ISE is user certificate authentication. This approach typically involves the use of digital certificates, which are managed by a Public Key Infrastructure (PKI). In this case, the Cisco ISE interacts with an external certificate authority to validate and manage those certificates for authentication purposes.

User certificate authentication enhances security by leveraging the encryption and identity verification capabilities that certificates provide. When a client device authenticates using a certificate, it presents its digital certificate, and Cisco ISE validates it against the external database (such as a Certificate Authority). This ensures that only devices with valid, trusted certificates are granted access to the network.

In contrast, other methods like MAC Authentication Bypass (MAB), 802.1X user password authentication, and web authentication either do not require an external database or can operate independently from one. MAB relies on the MAC addresses of devices to allow access without the need for an external authentication method. 802.1X user password authentication uses usernames and passwords, which can be managed by internal databases or simple configurations, while web authentication generally employs HTTP and does not necessitate an external database for validating users.