CCNA QoS Interview Questions: Marking, Queuing, Policing and Shaping Explained

Key takeaways

• QoS only matters when there is congestion. It does not create bandwidth. It decides who waits or drops first.
• Voice is sensitive to delay, jitter and loss. Common targets: one-way delay under 150 ms, jitter under 30 ms and loss under 1 percent.
• Remember the sequence: classify, mark, queue and schedule, then police or shape. Mark close to the source, at the trust boundary.
• DSCP EF (46) is the usual marking for voice. AF41 (34) is common for interactive video. CS3 (24) is often call signalling.
• Policing drops or re-marks excess traffic. Shaping delays it with buffers and smooths the rate.

What is QoS and why is it needed?

Quality of Service is the set of tools that give different traffic different treatment, so important or delay-sensitive applications keep working when a link is congested. On an idle network, QoS does nothing visible. When a link fills up, QoS decides which packets are queued, delayed or dropped first. CCNA 200-301 expects you to explain the per-hop behaviour concepts: classification, marking, queuing, congestion handling, policing and shaping. The Cisco certifications page lists the current exam topics.

What are delay, jitter and packet loss?

Commonly quoted guidelines for voice are one-way delay of 150 ms or less, jitter of 30 ms or less and loss of 1 percent or less. These are widely used planning numbers; check them against your vendor's design guide.

Why does voice traffic need QoS?

Voice packets are small, sent at regular intervals and cannot be retransmitted usefully, because a late packet is a lost packet. A large file transfer sharing the same link can fill queues and delay voice packets. QoS puts voice into a priority queue so it is sent first, within limits.

What are the QoS steps?

1. Classification: identify the traffic, by ACL, protocol, port, existing marking or application recognition.
2. Marking: set a value in the packet so other devices can treat it consistently. Layer 2 uses CoS (802.1p, 3 bits in the VLAN tag). Layer 3 uses DSCP (6 bits in the IP header, in the ToS or Traffic Class field).
3. Queuing and scheduling: place packets in queues and decide the order they are sent.
4. Congestion avoidance: drop some packets early (for example WRED) to slow TCP senders before queues overflow.
5. Policing and shaping: enforce or smooth traffic rates.

Which DSCP values should you know?

What is a trust boundary?

The trust boundary is where a device decides whether to believe markings from the sender. Typically you trust markings from an IP phone but not from a user PC, so the access switch re-marks or trusts selectively. Marking as close to the source as possible, then trusting it inside the network, keeps policies simple.

What queuing methods do interviewers ask about?

• FIFO: first in, first out, no differentiation.
• WFQ and CBWFQ: weighted fair queuing, with class-based configuration giving each class a minimum bandwidth.
• LLQ (low latency queuing): CBWFQ plus a strict priority queue for voice. Priority traffic is policed so it cannot starve other classes.

What is the difference between policing and shaping?

What does a basic QoS configuration look like?

Cisco IOS uses the Modular QoS CLI (MQC): class-map to classify, policy-map to act, service-policy to apply. The example is for study in a lab, and values must match your own design.

class-map match-any VOICE
 match dscp ef
class-map match-any SIGNALING
 match dscp cs3
!
policy-map WAN-OUT
 class VOICE
 priority percent 20
 class SIGNALING
 bandwidth percent 5
 class class-default
 fair-queue
!
interface GigabitEthernet0/0
 service-policy output WAN-OUT

Check it with show policy-map interface GigabitEthernet0/0, which shows matched packets and drops per class. To mark traffic from a phone VLAN on a switch you would use mls qos trust or platform-specific commands, which differ by switch family.

Sample interview questions to practise

1. A user complains calls are choppy only during file backups. How do you confirm and fix it?
2. Where would you mark traffic and why?
3. When would you shape and when would you police?
4. What does "priority percent 20" guarantee, and what does it limit?
5. How do you verify QoS works on an interface?

Next steps

To build and test these configurations, see WebAsha's CCNA training. More prep: CCNA interview questions and CCNA voice interview questions.

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