For some time now I have seen my Wireshark traces and the event log on the Mediation Server report an issue with regard to SIP Options. I finally was forced to pay attention to this item during a recent deployment.

In the image above the Source IP is the Lync FE and & are both gateways. Lync here sends the SIP Options Request and both gateways reply with OK.

What do SIP Options do?
They are used by Lync to determine if the gateway\SBC is good and ready to accept calls from Lync (no that’s not a typo, this is in relation to outbound calls).

Lync sends a SIP Options request to the Gateway and expects an answer (apparently any answer will do) usually a 200 OK. These messages are sent every 4 seconds. If no response is received for a period of up to 30 seconds the route using this particular gateway is marked as down. 

If its the only gateway Lync will continue to attempt to route calls across it. If there is an alternate gateway available then this will receive the outbound calls instead.

Sometimes you may come across a gateway that sends SIP Option messages to Lync, in this case Lync will stop sending Options messages and instead reply to the options messages it receives with an OK. Lync will also send a SIP Option messages every 20 seconds to be sure all is good.

If Lync stops seeing the SIP messages from the gateway it will revert to taking over the role of sending messages in 5 minutes from the last Options message received.