We ae using Policy based Routing based on destination ports and the TOS flags. The skype devices are set to mark signaling as CS3, voice as EF and video as AF41. The PbR matches the TOS field (0x60, 0x88 and 0xb with a mask of 0xFC).
In that way only the skype traffic is routed out the Skype interface. You need to set up Skype to mark the traffic properly. I'm not sure how his is done and probably depends on your Skype setup. Obviously the QoS markings will be ignored on the Internet.
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