Examkin

Question

Difficulty: MediumQoS Per-Hop Behaviors, Classification, Marking, Queuing, and Traffic Shaping/Policing

A network administrator is deploying Cisco IP Phones across an enterprise network where desktop PCs are daisy-chained to the secondary switch port on each IP Phone. Which configuration location represents the recommended placement of the QoS trust boundary to prevent PC traffic from spoofing high-priority Layer 2 CoS values while ensuring voice packets are appropriately prioritized?

  1. At the Cisco IP Phone, trusting CoS markings generated by the phone while untrusting and remarking traffic from the connected PC to CoS 0.Answer
  2. B
    At the access switch port, trusting all incoming CoS frames indiscriminately from both the IP Phone and the attached PC.
  3. C
    At the desktop PC network interface card (NIC), allowing the desktop operating system to mark data frames before transmission.
  4. D
    At the core switch interface, deferring all classification and trust boundary enforcement until traffic leaves the access tier.

Answer

The QoS trust boundary should be enforced at the Cisco IP Phone, trusting voice traffic tagged by the phone while untrusting and rewriting CoS markings from the attached PC to 0.
Extending the trust boundary to the Cisco IP Phone ensures that voice traffic generated by the phone is trusted, while traffic originating from the daisy-chained PC is untrusted and rewritten to a default best-effort value (CoS 0) before entering the switch access port.

Step-by-Step Solution

1
Identify the risk of untrusted endpoints at the network edge.
Desktop PCs connected to the IP phone can potentially send frames with altered Class of Service (CoS) bits to gain unauthorized priority.
If untrusted PC traffic is prioritized, critical voice traffic may experience jitter, latency, or packet loss.
2
Determine the optimal trust boundary enforcement point.
The trust boundary is extended to the Cisco IP Phone using switch commands (e.g., `mls qos trust cos` combined with `switchport priority extend trust` or `switchport priority extend cos 0`).
The Cisco IP Phone acts as a micro-switch, marking its own voice payload appropriately while stripping or remarking incoming traffic from the attached PC port down to CoS 0.

Key Concept

QoS Trust Boundary Placement and CoS Marking Enforcement
Rate this question

Topics

QoS Per-Hop Behaviors, Classification, Marking, Queuing, and Traffic Shaping/Policing

More questions from this topic