350-401 Question #193: Real Exam Question with Answer & Explanation

Question

Drag and Drop Question Drag and drop the Qos mechanisms from the left to the correct descriptions on the right. Answer:

Options

• CoSportion of the 802.1Q header used to classify packets
• DSCPportion of the IP header used to classify packets
• shapingbandwidth management technique which delays datagrams
• policingtool to enforce rate-limiting on ingress/egress
• policy mapmechanism to apply a QoS policy to an interface
• service policymechanism to create a scheduler for packets prior to forwarding

Explanation

QoS Mechanisms — Explained

1. CoS → portion of the 802.1Q header used to classify packets

Class of Service (CoS) operates at Layer 2. The 802.1Q VLAN tag contains a 3-bit Priority Code Point (PCP) field (also called CoS), providing 8 priority levels (0–7). Because it lives in the Ethernet frame header, it only works on switched/VLAN-tagged traffic — it disappears when a packet crosses a Layer 3 boundary.

2. DSCP → portion of the IP header used to classify packets

Differentiated Services Code Point (DSCP) operates at Layer 3. It uses the 6 most significant bits of the ToS (Type of Service) byte in the IP header, providing 64 possible values. Because it's in the IP header, it survives Layer 3 routing — making it the preferred marking mechanism end-to-end across routed networks.

Common mistake: Confusing DSCP with IP Precedence. IP Precedence uses only the first 3 bits of the same ToS byte; DSCP is its modern, more granular replacement and is backward-compatible.

3. Shaping → bandwidth management technique which delays datagrams

Traffic shaping buffers excess traffic and smooths the output rate by introducing delay. Packets exceeding the configured rate are held in a queue and sent later rather than dropped. This produces a smooth, conforming traffic profile at the cost of added latency.

Common mistake: Thinking shaping is lossless — queues can still overflow and drop packets if sustained traffic far exceeds the shape rate.

4. Policing → tool to enforce rate-limiting on ingress/egress

Traffic policing measures the traffic rate against a configured limit and immediately drops or re-marks non-conforming packets — no buffering, no delay. It's applied at both ingress and egress and is the standard tool ISPs use to enforce customer SLAs.

Common mistake: Confusing policing with shaping. The key distinction: policing drops or re-marks; shaping delays. Policing has no queue for excess traffic.

5 & 6 — Note: These two descriptions appear swapped in the question

In standard Cisco MQC (Modular QoS CLI) terminology:

The question maps them as:

• policy map → applies a QoS policy to an interface
• service policy → creates a scheduler for packets prior to forwarding

This is reversed from standard Cisco IOS behavior. If this is a Cisco exam question, flag it — the conventional understanding is:

• policy-map = the scheduler definition (what to do with traffic classes)
• service-policy = the interface attachment command (where to apply the policy-map)

Common mistake: Exactly what this question demonstrates — confusing which command creates the policy vs. which applies it. Remember: you define with policy-map, you deploy with service-policy.

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