In Wi-Fi Cloud, what is the third-radio used for on tri-radio APs?

The third-radio on tri-radio access points (APs) is utilized as a Wireless Intrusion Prevention System (WIPS) sensor. This setup enables the AP to automatically detect and respond to wireless threats, thereby enhancing the security of a Wi-Fi network.

By dedicating a radio specifically for WIPS, organizations can continuously monitor the wireless environment without interfering with the performance of the primary radios that are handling user traffic. This dedicated sensor can identify rogue access points, monitor for unauthorized devices, and detect potential attacks, allowing for proactive security measures.

In the context of the other choices, while options like an additional 5 GHz radio or guest SSID broadcasting could enhance user experience, these functions do not align with the security focus of the third-radio. The captive portal option serves a different purpose, primarily for authentication and access control, and is not related to intrusion detection or prevention like the WIPS sensor is.