Rtp multicast with SIP

[m.mz]

We have a sip application more or less like a SIP User Agent, but using RTP multicast for conference. We are using a CISCO 3745 with different voice interfaces (FXS, FXO, E&M, ISDN BRI, E1) to provide our application to talk to external world. The problem is when we send INVITE SIP with sdp rtp multicast in the c field (connection information), the Cisco Router alaways sent me back a sdp rtp unicast. Is there any way to configure the router to send me back a rtp multicast? If there´s no way, how could I deal to get conference with other users (FXS, FXO etc)?

[irisrios]

As per the document below, Cisco IOS version 12.1T doesn't support multicast RTP sessions . May be for the version of IOS you have, you could check out with the features in the Software Advisor tool.

SIP Transport Layer Protocols

http://www.cisco.com/en/US/products/sw/iosswrel/ps1834/products_feature_guide09186a008007fef0.html#xtocid319335