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Zorluk: ZorQoS Per-Hop Behaviors, Classification, Marking, Queuing, and Traffic Shaping/Policing

An enterprise network administrator is deploying Quality of Service (QoS) across access switch ports where desktop PCs are daisy-chained through Cisco IP Phones. Which two statements accurately describe Layer 2 and Layer 3 QoS field characteristics and trust boundary enforcement in this design?

  1. The 3-bit Class of Service (CoS) priority field is located in the IEEE 802.1Q header tag and is stripped when frames cross a Layer 3 routed boundary.Cevap
  2. Extending a trust boundary to the Cisco IP Phone allows the switch to accept voice CoS values from the phone while re-marking untrusted data traffic from the PC to CoS 0.Cevap
  3. C
    Traffic policing configured on ingress access interfaces buffers non-conforming packet bursts until interface queue congestion clears.
  4. D
    Traffic policing on switch ingress queues delays out-of-profile voice packets in a dedicated Priority Queue (PQ) to prevent packet loss.

Cevap

The two correct statements are that the 3-bit Class of Service (CoS) field resides within the IEEE 802.1Q header tag and is stripped when traversing a Layer 3 boundary, and extending the trust boundary to a Cisco IP Phone allows trusting phone voice markings while re-marking untrusted PC traffic to CoS 0.
Layer 2 Class of Service (CoS) consists of 3 bits in the 802.1Q header tag, which is removed whenever a router decapsulates an Ethernet frame at a Layer 3 boundary. Additionally, extending a trust boundary to a Cisco IP Phone ensures that voice traffic from the phone retains high priority while untrusted PC frames passing through the phone are reset to CoS 0 / DSCP 0.

Adım Adım Çözüm

1
Analyze Layer 2 vs Layer 3 QoS header field encapsulation
CoS is a 3-bit field (Priority Code Point / PCP) inside the 802.1Q VLAN tag. Because Layer 2 tags are stripped by a router at a L3 hop, CoS markings do not survive routing unless translated into 6-bit L3 DSCP markings within the IP header.
Differentiates L2 link-specific marking from L3 end-to-end IP marking.
2
Evaluate Cisco IP Phone trust boundary behavior
Using commands such as `mls qos trust device cisco-phone`, the switch trusts the CoS/DSCP generated by the phone itself but forces untrusted PC traffic arriving at the phone's switch port down to CoS 0.
Prevents end-user PCs from spoofing high-priority QoS markings to gain unauthorized bandwidth prioritization.
3
Differentiate Traffic Policing mechanics from Traffic Shaping and Queuing
Policers measure traffic against a CIR and enforce rate limits by dropping or remarking excess frames immediately without buffering. Buffering excess burst packets is exclusively performed by shapers.
Refutes distractors that claim policing buffers or queues non-conforming traffic.

Anahtar Kavram

QoS Trust Boundaries, CoS vs DSCP Header Lifecycles, and Policing Mechanics
Tahmini Süre:2m 0s
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Konular

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

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