Cisco IP Phone Trust Boundary PC Packet Handling
Short answer
B The phone does not mark any packets, and the access switch sets up the CoS values.
Question
Key Takeaways
The exam tests the distinction between trusted voice traffic and untrusted data traffic; the common trap is assuming the phone automatically marks all incoming traffic or ignores the trust boundary concept entirely.
This question tests the behavior of a Cisco IP phone acting as a trust boundary for traffic originating from an attached PC. The community consensus is that the phone does not mark CoS values for untrusted PC traffic, leaving the access switch to handle classification and marking.
Common mistake: Option C is the most common wrong answer because candidates assume the phone re-marks PC traffic to default values (0/3/5). However, without explicit configuration to trust and remark PC traffic, the phone typically leaves it alone, relying on the switch.
Expert Analysis
Why the Answer Is Correct
When a trust boundary is established at the IP phone, the phone trusts its own voice traffic but treats traffic from the connected PC as untrusted. Consequently, the phone does not apply QoS markings (CoS/DSCP) to packets received from the PC by default. Instead, these unmarked packets are sent to the access switch port, which is configured to trust the phone's control protocol but must classify and mark the data traffic coming from the PC.
Why the Other Options Are Wrong
Option A is incorrect because the phone only marks its own voice signaling and media; it does not arbitrarily set CoS 3/5 for PC traffic. Option C is a distractor based on the misconception that the phone actively resets PC traffic to specific defaults; in reality, it usually passes them through unmarked unless explicitly configured otherwise. Option D is incorrect because QoS marking typically occurs at the access layer (the edge), not pushed up to the distribution layer for initial classification.
Community Comment Notes
Comment [1] correctly explains that the switch enforces CoS values for PC traffic since the phone doesn't mark it. Comment [4] provides technical nuance, noting that if frames are tagged, CoS might be reset to zero, reinforcing why the switch needs to handle the marking logic.
Community Votes
71% of anonymous learners picked answer B. 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
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R4m0nECPC 👍 4 Answered BIn a Cisco IP phone deployment with a trust boundary on the phone, the phone typically does not trust the traffic coming from a connected PC. As a result, the phone does not mark the CoS (Class of Service) values for packets received from the PC. Instead, the access switch, which is typically configured to trust the phone's markings, will set or enforce the appropriate CoS values based on the network's QoS policies.
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cb586d3 👍 2 Answered C"we won’t remark any packets or Ethernet frames anymore at the access layer switch. The IP phone will mark all traffic. Note that the computer is outside of the QoS trust boundary. This means that we don’t trust the marking of the computer. We can remark all its traffic on the IP phone if we want."
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b3532e4 👍 11. That is correct that there is no CoS field if the phone is receiving untagged frames from the PC. If the frames are tagged, the CoS value will be reset to zero. 2. Yes. The DSCP of traffic received from the PC is also re-marked to zero by default. Therefore, 'trust dscp' should be fine on your switch port. Also, you might want to configure 'mls qos trust device cisco-phone' so that DSCP is only trusted when a Cisco IP phone is plugged into the port.
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decdca7 👍 1 Answered BThe phone is trusted and the phone will mark the packets
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