Final answer:
The protocol used to encapsulate EAP data for 802.1X authentication is RADIUS, which serves to securely transport authentication information between the authenticator and the authentication server.
Step-by-step explanation:
The protocol used to encapsulate the Extensible Authentication Protocol (EAP) data between the authenticator and the authentication server during 802.1X authentication is Radius (Remote Authentication Dial-In User Service). RADIUS is commonly implemented in wireless networks and other types of network access control implementations to provide a centralized authentication and accounting service. In the context of 802.1X, the authenticator acts as a go-between for the client device seeking access and the RADIUS server that verifies the user's credentials. The basic role of RADIUS in this setup is to encapsulate and transport the EAP packets from the authenticator to the authentication server and back. This allows for secure transmission of authentication information over the network.