When a link is congested, Quality of Service decides which packets get the bandwidth and which wait. This chapter introduces the trust boundary, classification with DSCP, and the queueing behaviour you should expect on a router.
On an idle interface every packet leaves immediately and QoS has no effect. The moment offered traffic exceeds line rate the router queues packets. QoS is the policy that decides which queue each packet enters and the order packets are dequeued.
Endpoints set their own DSCP. Treat that markings as untrusted at the edge — re-mark or trust only as policy allows. The ideal trust boundary is the access switch port: trust DSCP from a known IP phone or untrusted user, policy-enforce everything else.
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