QoS Fundamentals Quick Reference

Classification decides which traffic is which, using ACLs, existing DSCP/CoS values, or application signatures as match criteria.

Marking stamps that classification onto the packet — almost always as a DSCP value in the IP header — so downstream devices can act on it without re-classifying.

The trust boundary is where you decide whether to honor markings already present on incoming traffic, or re-classify and re-mark instead. Set it as close to the source as operationally practical, and never trust markings from a port an untrusted end user can directly reach.

Queuing determines the order packets leave a congested interface. Congestion management is what prevents indiscriminate tail-drops when an interface is oversubscribed.

Policing and shaping both enforce a bandwidth limit, but respond to excess traffic very differently — see the comparison table below.

It's rarely a missing policy-map. It's almost always a correctly written policy-map that either was never applied to an interface with 'service-policy', or is matching on a DSCP value that never actually arrives because the trust boundary upstream didn't honor or apply it.

Always verify with 'show policy-map interface' and check for non-zero matched packets in the class you expect traffic in — a policy that looks right in the running-config isn't the same as a policy that's actually matching real traffic.