I have just finished watching one of the Cisco Press "Meet the Aouther" webinar WRT regard "QoS in Campus Networks" (and last week QoS Design Best Practices and Strategies) and along with this Cisco document (http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/uc_system/design/guides/videodg/vidguide/qos.html) WRT QoS in Video and Telepresence environments. There is a paragraph which says:
"QoS should be configured on the Cisco TelePresence Video Communication Server (VCS) because it processes the media and the call signaling. The endpoints that register to the VCS (such as the Cisco TelePresence System EX Series, C Series, Cisco IP Video Phone E20, or others) should be configured so that the call signaling uses DSCP CS3 and the media from those endpoints is marked as DSCP AF41."
The VCS has only ONE DSCP value that is applied to both media and signaling. In this case, and if DSCP tags are applied at the endpoint (and assuming that these tags are not altered elsewhere) will the VCS simply overwrite everything or honour what has come before it? I would probably have set the VCS DSCP tag to CS4, but I'n not enterly sure if this is the most prudent course of action.
Thanks Patrick. Indeed, we have settled on CS4 for our videoconferencing media flows and CS3 for our signalling simply as the organisations we manage do no utilise actual Telepresence systems (and are unlikely to do so), and we want to distinguish our traffic from other interactive video such as web conferencing. We would then recommend that the priority queue would be set for EF+CS4+CS5 traffic, with the next queue looking after the likes of CS3 and AF41 traffic.
However, this still dosen't quite answer the question with regard to whether the VCS actually overwrites the tags on flows through it (if already set) or leaves pre-tagged traffic alone. As the VCS has only a single global QoS value, I assume that if it were set to CS4 (DSCP value 32), then if the endpoint were actually set with CS4 on the media and CS3 on the signalling, then the VCS would actually overwrite the signalling packet headers. This then seems a bit pointless that you can set different QoS tags to different traffic at the endpoint (or elsewhere for that matter) only for them to be overwritten.
The administration guide makes no indication as to what the VCS will actually do, just that the single global value will be applied to both media and signalling traffic.
I agree the VCS admin guide isn't very clear on this subject. I was just providing you with what some other organizations have done, though in ours we use AF41 across the board, unlike you where you might use difference QoS values for different things.