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QoSPrivileged EXEC

show mls qos

Displays the QoS (Quality of Service) configuration and statistics on a Cisco switch, used to verify and troubleshoot QoS policies.

Syntax·Privileged EXEC
show mls qos

When to Use This Command

  • Verify that QoS is enabled globally on the switch after configuration.
  • Check the default QoS trust state and port settings on a switch.
  • Troubleshoot QoS issues by reviewing queue statistics and drop counters.
  • Audit QoS configuration before deploying changes in a production network.

Command Examples

Basic QoS Status Display

show mls qos
QoS is enabled

  QoS global counters:
    Total packets: 123456
    IP packets: 98765
    Non-IP packets: 24691
    Bridged packets: 50000
    Multicast packets: 10000

  QoS trust state: untrusted

  QoS statistics:
    Total drops: 0
    Queue drops: 0
    Policing drops: 0
    Marking statistics:
      Marked packets: 0
      Unmarked packets: 123456

Line 1: 'QoS is enabled' confirms QoS is globally active. 'Total packets' shows all packets processed by QoS. 'IP packets' and 'Non-IP packets' break down by type. 'Bridged packets' are layer2 switched. 'Multicast packets' are multicast traffic. 'QoS trust state: untrusted' means the switch does not trust incoming CoS/DSCP markings. 'Total drops' shows packets dropped due to QoS. 'Queue drops' are drops from egress queues. 'Policing drops' are packets dropped by policers. 'Marking statistics' show how many packets were marked or left unmarked.

QoS with Trust and Policing

show mls qos
QoS is enabled

  QoS global counters:
    Total packets: 500000
    IP packets: 400000
    Non-IP packets: 100000
    Bridged packets: 200000
    Multicast packets: 50000

  QoS trust state: trust dscp

  QoS statistics:
    Total drops: 1500
    Queue drops: 1000
    Policing drops: 500
    Marking statistics:
      Marked packets: 2000
      Unmarked packets: 498000

Here 'QoS trust state: trust dscp' indicates the switch trusts DSCP values from incoming packets. 'Total drops: 1500' shows significant drops, with 'Queue drops: 1000' and 'Policing drops: 500'. This suggests congestion or policing is active. 'Marked packets: 2000' shows some packets were re-marked, possibly by a policy.

Understanding the Output

The 'show mls qos' output provides a high-level view of QoS operation on a Catalyst switch. The first line indicates whether QoS is globally enabled. The 'QoS global counters' section shows total packets processed, broken down by IP, non-IP, bridged, and multicast. This helps understand traffic mix. The 'QoS trust state' line is critical: 'untrusted' means the switch ignores any markings and applies default CoS (usually 0); 'trust dscp' or 'trust cos' means the switch preserves and uses the incoming markings for queueing. The 'QoS statistics' section shows drop counters: 'Total drops' is the sum of all QoS-related drops; 'Queue drops' indicate egress congestion; 'Policing drops' indicate traffic exceeding configured policers. High drop counts suggest misconfiguration or insufficient bandwidth. 'Marking statistics' show how many packets were marked (e.g., by a policy) versus unmarked. In a healthy network, drops should be minimal or zero, and trust state should match the administrative domain boundary.

CCNA Exam Tips

1.

CCNA exam tip: Remember that QoS is disabled by default on most Catalyst switches; you must enable it globally with 'mls qos'.

2.

CCNA exam tip: The trust state (untrusted vs trust dscp/cos) is a common exam topic; understand that untrusted resets markings to 0.

3.

CCNA exam tip: 'show mls qos' does not show per-interface details; use 'show mls qos interface <int>' for that.

4.

CCNA exam tip: High queue drops indicate congestion; consider shaping or increasing bandwidth.

Common Mistakes

Mistake: Forgetting to enable QoS globally with 'mls qos' before applying policies; the switch will not enforce QoS.

Mistake: Assuming 'show mls qos' shows per-interface trust; it only shows global trust; use interface-specific commands.

Mistake: Misinterpreting 'trust dscp' as automatically applying QoS policies; trust only preserves markings, policies must be applied separately.

Related Commands

show policy-map

Displays the configuration and statistics of all policy maps applied to interfaces, used to verify QoS policies and monitor traffic class counters.

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