350-801 Question #447: Real Exam Question with Answer & Explanation

Question

If a trust boundary is placed on a Cisco IP phone, what occurs with the packets that come to the phone from a PC?

Options

• AThe phone sets/resets the CoS field to 3 for voice signaling and 5 for voice traffic.
• BThe phone does not mark any packets, and the access switch sets up the CoS values.
• CThe phone sets/resets the CoS field to 0; voice signaling is set to 3; and voice traffic is set to 5.
• DQoS is performed at the distribution layer, and packets are marked at this boundary.

Explanation

When a Cisco IP phone is configured as a trust boundary, it resets the CoS field for data packets from a connected PC to 0, while correctly marking its own voice signaling as CoS 3 and voice traffic as CoS 5.

Common mistakes.

• A. The phone will not set PC data traffic to CoS 3 or 5; instead, it resets untrusted traffic from the PC to a CoS value of 0 to remove any potentially fraudulent markings.
• B. If the phone is established as the trust boundary, it actively marks its own voice packets with appropriate CoS values and resets CoS values for PC data packets, rather than leaving all marking solely to the access switch.
• D. QoS marking for endpoint-originated traffic should occur as close to the source as possible, typically at the access layer where the trust boundary is defined on the phone, not primarily at the distribution layer for initial marking.

Concept tested. Cisco IP phone QoS trust boundary behavior

Reference. https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/srnd/12x/cucm_srnd_12x/qos.html#Cisco_Reference.dita_a9591460-7058-45b9-9669-07f06798a65f

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