Configuring QoS Trust for Cisco Desk Phones on Catalyst Switches

350-801 By islever Editorial Last updated

Short answer

D Switch(config)# interface gigabitethernet2/0/1

Question

Which set of QoS commands must be configured on an interface of a catalyst switch so the ingress traffic can be trusted for a desk phone?
  1. Switch(config)# interface gigabitethernet2/0/1
  2. Switch(config)# interface gigabitethernet2/0/1
  3. Switch(config)# interface gigabitethernet2/0/1
  4. Switch(config)# interface gigabitethernet2/0/1 Correct Answer

Key Takeaways

This question tests device-aware QoS trust policies, trapping candidates who default to generic CoS or DSCP trust commands instead of the phone-specific variant.

Trusting ingress QoS markings for IP phones requires explicit endpoint verification via CDP or LLDP-MED. Community consensus confirms that mls qos trust device cisco-phone is the mandatory command, ensuring only validated voice devices inherit priority traffic handling.

Common mistake: Candidates often choose mls qos trust cos or dscp, which blindly accept all incoming priority markings and bypass critical security checks for untrusted data hosts.

Expert Analysis

Why the Answer Is Correct

The command mls qos trust device cisco-phone explicitly instructs the Catalyst switch to verify incoming frames via CDP or LLDP-MED before trusting their CoS values. This ensures only authorized Cisco IP phones receive priority treatment while preventing untrusted hosts from spoofing voice traffic markings.

Why the Other Options Are Wrong

Generic trust commands like mls qos trust cos or dscp lack endpoint validation, making them unsuitable for secure VoIP deployments. Option D contains a known typo in the dump mis instead of mls, but it remains the only functionally correct choice when interpreted as intended.

Community Comment Notes

Multiple voters confirmed D based on the exact command syntax, noting the typo issue. One comment linked to the official Cisco switching forum thread validating the configuration approach for enterprise voice environments.

Official Reference

https://community.cisco.com/t5/switching/mls-qos-trust-cos-and-device-cisco-phone/td-p/1311619

This link opens the official vendor documentation in a new tab.

Exam Strategy

Focus on keyword matching for VoIP scenarios; always prioritize device-aware trust commands over blanket CoS/DSCP configurations when IP phones are involved. Practice verifying CDP advertisement states in your simulation labs to prevent configuration mismatches during the exam.

Community Votes

D
100%

100% of anonymous learners picked answer D. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 4 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 4

  1. b3532e4 👍 2
    mls qos trust device cisco-phone Answer: D
  2. c6176b5 👍 1 Answered D
    It seems to be D https://community.cisco.com/t5/switching/mls-qos-trust-cos-and-device-cisco-phone/td-p/1311619
  3. JefferX 👍 1 Answered D
    mls qos trust device cisco-phone would be the only command accepted on the interface of a SW
  4. sneff91 👍 1 Answered D
    There's a typo in D (it has "mis" instead of "mls"), but it's otherwise correct.

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