QoSCCNA 200-301

VoIP Calls Choppy — QoS Not Prioritizing Voice Traffic

Presenting Symptom

VoIP calls are choppy with intermittent audio dropouts and jitter, especially during peak usage hours.

Network Context

A small branch office with 50 users connects to headquarters via a 10 Mbps MPLS WAN link. The network uses Cisco 4321 ISR routers and 2960X switches. VoIP traffic uses a separate VLAN (VLAN 10) with a Cisco IP Phone and PC daisy-chain setup. QoS is configured but voice quality degrades when data traffic spikes.

Diagnostic Steps

Check interface congestion and queue drops

show interfaces gigabitethernet 0/0/0 | include output drops|queue

  Output queue: 0/40 (size/max)
  5 minute input rate 8000000 bits/sec, 1000 packets/sec
  5 minute output rate 9000000 bits/sec, 1200 packets/sec
  0 output drops, 0 queue drops

If output drops or queue drops are non-zero, the interface is congested and packets are being dropped. This indicates QoS may not be prioritizing voice correctly.

Verify QoS policy applied to the WAN interface

show policy-map interface gigabitethernet 0/0/0

  Service-policy output: QOS-POLICY
    Class-map: VOICE (match-any)
      0 packets, 0 bytes
      5 minute offered rate 0 bps, drop rate 0 bps
      Match: ip dscp ef (46)
      Priority: 0 kbps, burst bytes 1500, b/w exceed drops: 0
    Class-map: class-default (match-any)
      123456 packets, 98765432 bytes
      5 minute offered rate 8000000 bps, drop rate 0 bps
      Match: any

If the VOICE class shows 0 packets or the priority queue is not configured with sufficient bandwidth, voice traffic is not being classified or prioritized. The priority queue should show packets matching DSCP EF.

Check classification markings on voice traffic

show class-map VOICE

  Class Map match-any VOICE (id 1)
    Match ip dscp ef (46)

Confirm that the class-map matches DSCP EF. If the match criteria are incorrect (e.g., wrong DSCP value), voice traffic won't be classified.

Verify trust boundaries on access switches

show mls qos interface gigabitethernet 0/1

  trust: dscp
  default COS: 0
  DSCP mutation map: default
  QoS is enabled globally
  QoS is enabled administratively

If trust is not set to 'dscp' or 'cos', the switch may re-mark voice packets to best-effort. Ensure the interface connected to the IP phone trusts the DSCP marking.

Root Cause

The WAN interface (GigabitEthernet 0/0/0) has a QoS policy applied, but the priority queue for voice is not configured with a bandwidth value, causing voice packets to be treated as best-effort and dropped during congestion. Additionally, the access switch ports do not trust the DSCP markings from the IP phones, re-marking voice packets to 0.

Resolution

On the router: configure the priority queue with sufficient bandwidth (e.g., 128 kbps for voice). On the access switches: enable QoS trust on ports connected to IP phones. Router config: policy-map QOS-POLICY class VOICE priority 128 Switch config: interface GigabitEthernet0/1 mls qos trust dscp

Verification

Run 'show policy-map interface gigabitethernet 0/0/0' and verify the VOICE class shows packets and no drops. Run 'show mls qos interface gigabitethernet 0/1' to confirm trust is set to dscp. Make a test VoIP call and verify no choppiness.

Prevention

1. Always configure a priority queue with adequate bandwidth for voice traffic. 2. Set trust boundaries at the access layer to preserve DSCP markings from IP phones. 3. Monitor interface utilization and queue drops proactively using SNMP or NetFlow.

CCNA Exam Relevance

On the CCNA 200-301 exam, QoS scenarios often appear as troubleshooting questions where you must identify why voice traffic is not being prioritized. Expect multiple-choice questions asking which command to use to verify QoS policy application or to fix misconfigured trust boundaries. Key fact: Voice traffic uses DSCP EF (46) and must be placed in a priority queue.

Exam Tips

Remember that the 'priority' command is used for real-time traffic like voice; 'bandwidth' is for data classes.

The 'show policy-map interface' command is the primary tool to verify QoS policy application and packet counts.

Trust boundaries must be set on access ports; otherwise, switches re-mark voice packets to best-effort.

Commands Used in This Scenario

show class-map

Displays the configuration and match criteria of all class maps or a specific class map, used to verify QoS classification rules.

show policy-map interface

Displays the current QoS policy applied to an interface, including per-class statistics such as packets matched, bytes, and actions taken, used to verify and troubleshoot QoS configurations.

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