Refer to the exhibit. An engineer receives a report that a client cannot authenticate via phone in a secure WLAN. The phone uses Layer 2 Security WPA2, and other clients can authenticate successfully.

An invalid username or password causes the RADIUS server to return an Access-Reject specific to that client's identity, which appears in the debug output as an authentication failure tied to that MAC address while all other clients with valid credentials continue to authenticate successfully.

A RADIUS misconfiguration on the WLC - such as a wrong shared secret, incorrect server IP, or wrong port - causes RADIUS transactions to fail or return errors, which can manifest as selective failures depending on which RADIUS server or policy is hit for that client type.

An expired or absent server certificate would cause EAP server validation to fail for all clients performing certificate-validating EAP methods, not selectively for a single phone while other clients authenticate without issue.

A general certificate services failure would affect all clients relying on certificate-based EAP across the WLAN, making it inconsistent with the symptom that other clients are authenticating successfully.

An incorrect EAP method on the client would prevent EAP negotiation from completing, but the debug output evidence pointing to an Access-Reject response identifies credential or RADIUS configuration errors as the more direct root cause.