Cisco Certified Network Associate (CCNA) 200-301 · Free study guide

Objective 4.7 — Explain QoS per-hop behavior

Quality of Service manages how a device treats traffic when resources are contended. It does not manufacture bandwidth or repair an application. Each hop classifies and queues according to its local policy.

Classification and marking

Classification identifies traffic from fields, interfaces, applications, or other context. Marking writes a value such as a DSCP signal so downstream devices can recognize intended treatment without repeating deep classification. A trust boundary decides which incoming markings are accepted; untrusted endpoints should not grant themselves priority.

Queuing and congestion

Packets wait in queues when the outgoing interface cannot transmit them immediately. A scheduler decides which queue receives service and in what share or priority. Congestion creates delay and can cause drops when buffers fill. Priority treatment for delay-sensitive traffic needs a limit so it cannot starve other classes.

Policing

Policing measures a traffic rate against a policy and drops or remarks excess traffic. It does not buffer excess for later transmission. The result can be abrupt loss, which upstream TCP may interpret as congestion. Policing is useful for enforcing a boundary but can hurt applications when set below legitimate bursts without care.

Shaping

Shaping buffers excess traffic and releases it at a target rate, smoothing bursts at the cost of added delay. It is commonly applied outbound. The buffer is finite, so sustained excess can still drop. Shaping can make a sender conform to a slower provider rate more gracefully than immediate policing.

Worked scenario

A 100-Mb/s branch interface feeds a provider service policed at 50 Mb/s. Bursts leave the router at line rate and are dropped by the provider. Outbound shaping toward 50 Mb/s can queue and pace the burst locally. Voice can receive an appropriately bounded priority queue. Marking alone would not slow transmission.

Per-hop consistency

A DSCP mark has no guaranteed effect unless each downstream hop maps it to policy. Document classification, trust, marking, queue, and bandwidth behavior across the path. Wireless and WAN segments may translate markings into different mechanisms.

Verification evidence

Inspect policy attachment and direction, class match counters, offered and dropped rates, queue depth, scheduling, policer violations, and shaper statistics under representative load. Confirm application latency, jitter, and loss rather than declaring success from a configured policy.

Common traps

• Saying QoS adds bandwidth.
• Confusing marking with queuing.
• Saying a policer delays excess traffic.
• Saying a shaper never drops.
• Trusting endpoint priority markings automatically.

Readiness checklist

• I can order classification, marking, queuing, and scheduling.
• I distinguish policing from shaping.
• I understand per-hop treatment and trust boundaries.
• I verify counters and user-facing performance.