You find yourself unable to federate with a specific organisation in Lync. The remote contact appears as “Presence Unknown” even after you have allowed the federated domain in the Lync Control Panel. The next step is to enable Lync client logging, attempt to communicate with the affected user and then review the log. To view the log file you will need to use Snooper which can be found in the Lync resource kit. After opening the log with Snooper, select the messages tab and search for the remote contact. In this particular case the below error was received:
ms-diagnostics: 1047;reason="Failed to complete TLS negotiation with a federated peer server";WinsockFailureCode="10054(WSAECONNRESET)";WinsockFailureDescription="The peer forced closure of the connection”
The remote partners Lync/OCS Edge server did not trust the CA that issued certificate assigned to our Edge Server. To resolve this, the remote partner needed to install the root CA of our Edge Server, on their Edge server in the Trusted Root Certification Authorities Store.