QoSCCNA 200-301

QoS Policy Configured But Not Applied to Interface

Presenting Symptom

Voice traffic experiences high jitter and packet loss on an interface, even though a QoS policy has been configured globally.

Network Context

A small branch office with a Cisco 4321 ISR router running IOS XE 16.9 connects to the WAN via a GigabitEthernet0/0/0 interface. The router has a QoS policy for voice traffic (class-map matching dscp ef) with priority queuing, but the policy is not applied to the interface. The network uses a single VLAN for data and voice, with a Cisco 2960 switch at the access layer.

Diagnostic Steps

Check if the QoS policy is applied to the interface

show running-config interface gigabitethernet 0/0/0

interface GigabitEthernet0/0/0
 ip address 192.168.1.1 255.255.255.0
 duplex auto
 speed auto
! No service-policy output or input statement

If no 'service-policy' line appears, the policy is not applied. Normal output would include 'service-policy output VOICE-POLICY'.

Verify the QoS policy exists in the running configuration

show running-config | section policy-map

policy-map VOICE-POLICY
 class VOICE
  priority percent 10
 class class-default
  fair-queue

Confirms the policy-map is configured. If not present, the policy was never created.

Check interface statistics for QoS drops

show policy-map interface gigabitethernet 0/0/0

GigabitEthernet0/0/0

  Service-policy output: (not applied)

  No service policy is currently applied to this interface

If the output says 'not applied' or 'No service policy', the policy is not active. Normal output would show the policy name and per-class statistics.

Confirm the class-map used in the policy is correctly matching traffic

show class-map VOICE

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

Verifies the class-map matches DSCP EF. If the match criteria are wrong (e.g., missing or incorrect), traffic won't be classified.

Root Cause

The QoS policy-map 'VOICE-POLICY' was created globally but never applied to the GigabitEthernet0/0/0 interface using the 'service-policy output' command. Without application, the interface uses default FIFO queuing, causing voice packet drops during congestion.

Resolution

Apply the QoS policy to the interface in the correct direction (output for outbound traffic): configure terminal interface gigabitethernet 0/0/0 service-policy output VOICE-POLICY end This attaches the policy to the interface, enabling priority queuing for voice traffic.

Verification

Run 'show policy-map interface gigabitethernet 0/0/0' and verify output shows: GigabitEthernet0/0/0 Service-policy output: VOICE-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: 10% (10000 kbps), burst bytes 250000, b/w exceed drops: 0 Class-map: class-default (match-any) 0 packets, 0 bytes 5 minute offered rate 0 bps, drop rate 0 bps Match: any queue limit 64 packets (queue depth/total drops/no-buffer drops) 0/0/0 (pkts output/bytes output) 0/0 Also verify voice quality improves (e.g., reduced jitter via 'show call active voice brief').

Prevention

["Always apply QoS policies to interfaces immediately after creation to avoid misconfiguration.","Use a configuration template or automation to ensure consistent policy application across all WAN interfaces.","Implement monitoring (e.g., SNMP) to alert when interfaces have no QoS policy applied."]

CCNA Exam Relevance

On the CCNA 200-301 exam, this scenario tests understanding of QoS policy application. Expect multiple-choice or drag-and-drop questions asking which command applies a policy to an interface, or troubleshooting where the policy is missing. Key fact: 'service-policy {input|output} policy-map-name' must be used under the interface.

Exam Tips

Remember that 'service-policy' is applied under the interface, not globally.

The direction (input/output) must match the traffic flow; for outbound queuing, use 'output'.

Know that 'show policy-map interface' is the primary verification command for applied policies.

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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