IP Services // Lesson 07

Quality of Service.

Explain per-hop QoS behaviour including classification, marking, queuing, congestion management, policing and shaping.

ClassificationMarkingQueuingPolicingShaping
01 // Mental model

Start with the big picture.

Identify traffic using properties such as addresses, ports, protocol or existing markings.

CCNA focus: QoS manages competition during congestion; it does not create bandwidth. Preserve a trusted marking boundary and apply consistent per-hop behaviour.
02 // Building blocks

Know what each part does.

01

Classify

Identify traffic using properties such as addresses, ports, protocol or existing markings.

02

Mark

Write a value such as DSCP so downstream devices can recognise the traffic class.

03

Queue

Decide which waiting packets transmit first when an interface is congested.

04

Police or shape

Police drops/remarks excess traffic immediately; shaping buffers excess to smooth the rate.

03 // Compare and recognise

Read the clues.

ItemWhat to remember
ClassificationAnswers: what kind of traffic is this?
MarkingAdds a reusable label, commonly in the IP DSCP field.
Congestion avoidanceDrops selected traffic before a queue is completely full.
PHBThe forwarding treatment a device applies to a marked class at each hop.
04 // Packet treatment path

Packet treatment path.

Packet arrives
  -> classify application/flow
  -> mark or trust DSCP
  -> place into an egress queue
  -> schedule queues during congestion
  -> police or shape where policy requires
  -> transmit to the next hop

Read the example from top to bottom, then verify the resulting state. Configuration is only complete when the output matches the intended design.

05 // Exam and troubleshooting

Turn facts into a method.

  • Answers: what kind of traffic is this?
  • Adds a reusable label, commonly in the IP DSCP field.
  • Drops selected traffic before a queue is completely full.
  • The forwarding treatment a device applies to a marked class at each hop.
Exam checkpoint: QoS manages competition during congestion; it does not create bandwidth. Preserve a trusted marking boundary and apply consistent per-hop behaviour.
06 // Check yourself

Quality of Service quiz.

1. What does classification do?

2. Which field commonly carries an IP QoS marking?

3. What is the key difference between policing and shaping?

4. When is queuing most relevant?

5. Does QoS create additional link capacity?

Score: 0 / 5