Refer to the exhibit.

The data path being down while the control path is up indicates an issue with the data plane security configuration. The data-dtls setting on the AireOS 5520 WLC is responsible for enabling Datagram Transport Layer Security (DTLS) for the data plane, which encrypts the data traffic between the controllers. If this setting is disabled, the data path cannot be established securely, leading to the observed issue. Therefore, enabling data-dtls on the AireOS 5520 WLC is necessary to resolve the data path downtime.