The user's device fails 802.1X EAP-TLS authentication, but the client-side certificate is known to be valid. We need two likely causes.

EAP-TLS Process:Involves mutual certificate validation and TLS handshake between client and RADIUS server (proxied by NAD).

Causes (Client Cert OK):

Server Certificate Issues: Client doesn't trust server cert (Untrusted CA, name mismatch, expired).

EAP Type Mismatch:Client supplicant configured for different EAP type than RADIUS server policy.

RADIUS Server Issues:Policy misconfiguration, user not found, internal errors.

NAD <-> RADIUS Communication Failure:Switch cannot reach RADIUS server (IP connectivity, firewall, routing), incorrect shared secret.

Client Supplicant Misconfiguration:Incorrect identity, settings other than the certificate itself.

Network packet loss.

Analysis of Options (Select Two):

A: Wrong gateway affects L3 post-authentication.

B: ACL blocking EAPoL/RADIUS is possible but less common than config errors.

C:EAP-type mismatch:A very common configuration error leading to failure.

D: Wrong MAC address is irrelevant for EAP-TLS failure itself.

E: NAD not able to communicate with DNS servers: DNS isn't directly involved in EAP-TLS. However, if interpreted more broadly asNAD not able to communicate with the RADIUS server(due to IP routing, firewall, or incorrect server address), this is a very common cause of failure.

Conclusion:An EAP-type mismatch (C) is a prime suspect when basic certificate validity is assumed. Failure of the Network Access Device (NAD - the switch) to communicate with the RADIUS server (E, interpreted broadly as RADIUS reachability) is another major category of failure causes.