The firewall intercepts the browser traffic per the Authentication policy rule and impersonates the original destination URL, issuing an HTTP 401 to invoke authentication. However, because the firewall does not have the real certificate for the destination URL, the browser displays a certificate error to users attempting to access a secure site. Therefore, use this mode only when absolutely necessary, such as in Layer 2 or virtual wire deployments.
The firewall intercepts unknown HTTP or HTTPS sessions and redirects them to a Layer 3 interface on the firewall using an HTTP 302 redirect to perform authentication. This is the preferred mode because it provides a better end-user experience (no certificate errors). However, it does require additional Layer 3 configuration. Another benefit of the Redirect mode is that it provides for the use of session cookies, which enable the user to continue browsing to authenticated sites without requiring re-mapping each time the timeouts expire. This is especially useful for users who roam from one IP address to another (for example, from the corporate LAN to the wireless network) because they won’t need to re-authenticate when the IP address changes as long as the session stays open.
If you use Kerberos SSO or NTLM authentication, you must use Redirect mode because the browser will provide credentials only to trusted sites. Redirect mode is also required if you use Multi-Factor Authentication to authenticate Captive Portal users.