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While building solid QoS configurations internally can help optimize VoIP service, it is not a panacea that ensures every call is perfect as an ISP could be operating with congested ports, which is unfortunately generally outside of locus of control. Though this article outlines QoS primarily in the vein of VoIP, it can be applied to the rest of your network. However, do this sparingly and do not try to micromanage traffic priority through DSCP as it will likely cause contrasting results from attempting to involve too many service levels. Keep things simple: determine what needs to be delivered consistently and with minimal loss and determine what is not a priority. Everything else will fall into the "best effort" bucket of priority.
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