The Mediation Server role in Lync Server 2013 is much like any other role in that it uses certificates both for communication to other servers and for client services. A single certificate is required for each server in the Mediation Server Pool. This certificate is very basic, and has the following requirement:
• Default—The default certificate is used for MTLS communications between servers, and for securing SIP signaling in client communications. The certificate should contain the pool name in the Subject field, each Mediation Server’s name as a subject alternative name.