350-801 · Question #151
What happens to voice packets from a Cisco 8845 IP phone in the QoS trust boundary?
The correct answer is B. The voice packets are classified by the phone, and the classification is accepted. In a Quality of Service (QoS) trust boundary, voice packets originating from a Cisco 8845 IP phone are classified by the phone itself. The access layer switch then accepts and trusts this classification, forwarding the packets with their established QoS markings.
Question
What happens to voice packets from a Cisco 8845 IP phone in the QoS trust boundary?
Options
- AThe voice packets are not trusted, and the access layer switch reclassifies the packets.
- BThe voice packets are classified by the phone, and the classification is accepted.
- CThe phone and access layer switch negotiate the classification of packets.
- DCisco UCM determines how the voice packets are classified.
How the community answered
(30 responses)- A3% (1)
- B93% (28)
- D3% (1)
Why each option
In a Quality of Service (QoS) trust boundary, voice packets originating from a Cisco 8845 IP phone are classified by the phone itself. The access layer switch then accepts and trusts this classification, forwarding the packets with their established QoS markings.
If the access layer switch is configured to trust the phone, it will accept the classification rather than reclassifying the packets.
Cisco IP phones are intelligent endpoints that can classify their own voice and video traffic with appropriate QoS markings (e.g., DSCP EF for voice, AF41 for video). When an access layer switch is configured for a QoS trust boundary on the port connected to the phone, it trusts these markings and passes the packets without reclassification, preserving the QoS prioritization.
The classification process is not typically a negotiation between the phone and the switch; the phone classifies, and the switch either trusts or reclassifies based on its configuration.
Cisco UCM manages call control and signaling but does not directly classify individual voice packets as they traverse the network; that is handled by the endpoints and network devices.
Concept tested: QoS trust boundary for IP phones
Source: https://www.cisco.com/c/en/us/td/docs/solutions/Enterprise/Voice_and_Video/IP_Phone_QoS/IP_Phone_QoS.html
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