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

27 questions

All types27 Multiple choice14 Multiple select6 Matching7

A network engineer is configuring Quality of Service (QoS) mechanisms on a Cisco router interface to control bandwidth usage. Which two behaviors are characteristic of traffic policing rather than traffic shaping? (Select two.)

Select all that apply

Dropping excess packets immediately when traffic exceeds the configured rate limit

Remarking the DSCP or CoS values of out-of-profile traffic to lower priority values

Buffering excess packets in interface queues to smooth out traffic bursts for later transmission

Applying queuing delays to ensure traffic fits within a configured egress shaper profile

Show answer & explanation

Answer: Dropping excess packets immediately when traffic exceeds the configured rate limit; Remarking the DSCP or CoS values of out-of-profile traffic to lower priority values

Answer

Traffic policing is characterized by dropping excess packets immediately when traffic exceeds the rate limit and remarking DSCP or CoS values of non-conforming packets.

Traffic policing enforces a hard rate limit by either dropping out-of-profile packets immediately or remarking their QoS headers (such as DSCP or CoS) to a lower priority class so they can be dropped downstream if congestion occurs.

Step-by-Step Solution

Identify the key operational difference between policing and shaping

Policing acts on instantaneous traffic rates (inbound or outbound) by either dropping or remarking non-conforming packets.

Policers do not use memory buffers to hold excess traffic, whereas shapers buffer traffic to smooth bursts.

Evaluate the choices based on policing mechanics

Immediate packet drops and packet remarking are valid policing actions. Buffering packets and adding queuing delay describe shaping.

Shaping retains packets in a queue to smooth egress rate, while policing enforces strict rate boundaries.

Key Concept

Traffic Policing vs. Traffic Shaping Mechanics

Estimated Time:45s

Question 22Question →

Match each Quality of Service (QoS) mechanism or Per-Hop Behavior (PHB) on the left to its corresponding operational characteristic or header specification on the right.

Click a left item, then click its matching right item

Show answer & explanation

Answer

Expedited Forwarding matches DSCP decimal 46 / Strict Priority Queue service; Assured Forwarding 41 matches DSCP decimal 34 / Class 4 low drop probability; Traffic Policing matches immediately dropping or remarking excess traffic without queuing; Traffic Shaping matches buffering excess traffic to smooth bursty egress rates.

Expedited Forwarding (EF) is standard DSCP 46 (010110) intended for real-time traffic requiring low latency/jitter via priority queuing. Assured Forwarding 41 (AF41) corresponds to DSCP 34 (100010) offering Class 4 high-priority bandwidth with a low drop probability. Traffic Policing measures rate limits and drops or remarks exceeding traffic immediately. Traffic Shaping regulates egress rate spikes by holding overflow packets in memory buffers.

Step-by-Step Solution

Identify Differentiated Services Code Point (DSCP) values and binary encodings for Per-Hop Behaviors (PHBs).

EF uses DSCP 46 (010110). AF41 uses class 4 (001) with low drop (01) plus trailing zero (100010 = 34).

DiffServ DSCP uses 6 bits in the IPv4 ToS field. EF guarantees strict priority service while AFxy defines class x (1-4) and drop preference y (1-3).

Differentiate traffic enforcement mechanisms based on memory buffer utilization.

Policing discards/remarks non-conforming packets without delay; Shaping delays non-conforming packets in egress buffers.

Policers do not support queuing/buffering and operate inbound or outbound, whereas shapers require queue buffers and operate outbound only.

Key Concept

QoS Per-Hop Behaviors (EF/AF) and Rate Enforcement (Policing vs Shaping)

Question 23Question →

A network administrator configures Low Latency Queuing (LLQ) on a Cisco WAN router egress interface using Modular QoS CLI (MQC). The policy map contains a `priority` command allocated $128\text{ kbps}$ for Expedited Forwarding (EF) voice traffic and a `bandwidth` command allocated $512\text{ kbps}$ for Assured Forwarding (AF31) transactional data. During a link congestion event, voice traffic arrives at a sustained rate of $200\text{ kbps}$ . Which statement accurately describes how the router processes the excess voice traffic beyond the allocated $128\text{ kbps}$ ?

The router police-drops or remarks the excess

72\text{ kbps}

of voice traffic to prevent starvation of the CBWFQ data queues.

The router buffers the excess

72\text{ kbps}

of voice traffic in RAM until link utilization drops below capacity.

The router dynamically reallocates unused bandwidth from the AF31 queue to transmit all

200\text{ kbps}

of voice traffic without packet loss.

The router automatically demotes the DSCP marking of all voice packets from EF (

46

) to AF11 (

10

) and places them into the default queue.

Show answer & explanation

Answer: The router police-drops or remarks the excess $72\text{ kbps}$ of voice traffic to prevent starvation of the CBWFQ data queues.

Answer

The router drops or remarks the excess voice traffic exceeding the allocated priority bandwidth limit during congestion to protect remaining queues.

Low Latency Queuing (LLQ) implements a strict priority queue for latency-sensitive traffic such as voice. To prevent a high-volume voice burst from monopolizing the interface and starving other CBWFQ queues during congestion, LLQ polices the priority queue to its allocated bandwidth limit (

128\text{ kbps}

). Any traffic offered above this rate (

72\text{ kbps}

) is policed (dropped or remarked), allowing remaining queues to receive their guaranteed bandwidth.

Step-by-Step Solution

Analyze the QoS queuing mechanism in use.

Low Latency Queuing (LLQ) combines a strict priority queue (`priority` command) with Class-Based Weighted Fair Queuing (CBWFQ) queues (`bandwidth` command).

LLQ is designed to service delay-sensitive traffic like voice before any other queue.

Evaluate the behavior of the strict priority queue under interface congestion.

To prevent a strict priority queue from starving remaining CBWFQ queues, MQC enforces an implicit or explicit policer on the `priority` class during congestion.

Without a policing mechanism on the strict priority queue, an overload of voice traffic would prevent data queues from ever being serviced.

Calculate the incoming rate versus the configured threshold.

Configured priority bandwidth =

128\text{ kbps}

; offered traffic rate =

200\text{ kbps}

. The excess volume is

200 - 128 = 72\text{ kbps}

Traffic up to

128\text{ kbps}

is transmitted with strict priority, and the remaining

72\text{ kbps}

is subjected to policing (dropped or remarked depending on explicit configuration).

Key Concept

LLQ Priority Queue Policing Mechanics under Congestion

Estimated Time:2m 0s

Question 24Question →

An enterprise WAN link connects a remote branch office to the primary data center over a service provider connection with a contracted Committed Information Rate (CIR) of $10\text{ Mbps}$ . The service provider immediately drops any bursts that exceed $10\text{ Mbps}$ at the ingress port of the provider edge router. To prevent packet drops during short bursts by queuing excess packets in memory and releasing them at a controlled rate, which Quality of Service (QoS) mechanism should be applied to the enterprise egress router interface?

Traffic shaping

Traffic policing

Layer 2 Class of Service (CoS) PCP marking

Differentiated Services Code Point (DSCP) remarking

Show answer & explanation

Answer: Traffic shaping

Answer

Traffic shaping

Traffic shaping uses internal memory buffers to store packets that exceed the configured bandwidth limit (such as a

10\text{ Mbps}

CIR) during burst periods. It then releases the queued packets onto the physical link at a steady, controlled rate, preventing packet drops at downstream provider interfaces that strictly enforce rate limits.

Step-by-Step Solution

Analyze the operational requirements of the WAN interface scenario.

The service provider enforces a strict limit (

10\text{ Mbps}

) and drops micro-bursts that exceed the CIR.

Enterprise traffic exceeding the contracted threshold gets dropped at the provider edge unless smoothed prior to egress.

Compare traffic conditioning mechanisms (policing vs. shaping).

Traffic policing discards out-of-profile packets or remarks them; traffic shaping buffers excess traffic and queues it for delayed transmission.

Buffering smoothed traffic prevents packet drops at the provider edge during brief micro-bursts.

Select the mechanism that buffers excess traffic for egress transmission.

Traffic shaping is the correct mechanism.

It aligns egress transmission rates with the contracted provider rate via packet buffering.

Key Concept

Traffic Shaping vs. Traffic Policing Mechanisms

Question 25Question →

Match each Quality of Service (QoS) concept or mechanism on the left to its corresponding operational characteristic or header specification on the right.

Click a left item, then click its matching right item

Show answer & explanation

Answer

Expedited Forwarding (EF) matches DSCP value 46; Class-Based Weighted Fair Queuing (CBWFQ) matches bandwidth allocation without strict priority; Traffic Policing matches dropping or remarking excess packets without buffering; Class of Service (CoS) matches the 3-bit Layer 2 802.1Q header field.

Each QoS concept correctly aligns with its protocol layer, header field, or operational mechanism: Expedited Forwarding (EF) maps to DSCP 46; CBWFQ provides minimum bandwidth guarantees across user-defined classes; Traffic Policing enforces rate bounds by dropping/remarking packets without using buffers; and CoS utilizes the 3-bit field inside IEEE 802.1Q headers.

Step-by-Step Solution

Identify the DSCP Per-Hop Behavior (PHB) designated for delay-sensitive voice payload.

Expedited Forwarding (EF) corresponds to DSCP value 46 (

101110_2

EF PHB ensures a guaranteed service rate with minimal delay and jitter for real-time applications.

Analyze how Class-Based Weighted Fair Queuing (CBWFQ) manages bandwidth.

CBWFQ allocates guaranteed minimum bandwidth percentages to defined classes.

Standard CBWFQ guarantees bandwidth per class during congestion but does not offer strict latency prioritization unless paired with Low Latency Queuing (LLQ).

Distinguish Traffic Policing from Traffic Shaping mechanics.

Policing enforces rate limits by immediately dropping or remarking excess packets without queue buffers.

Traffic shaping buffers excess traffic to smooth burst profiles, whereas policing acts instantaneously without memory buffering.

Evaluate Layer 2 QoS marking standards.

Class of Service (CoS) resides in the 3-bit Priority Code Point (PCP) field within an IEEE 802.1Q Ethernet tag.

CoS operates strictly at Layer 2 across trunk links and allows values from 0 through 7.

Key Concept

QoS Per-Hop Behaviors, Marking, Queuing, and Traffic Conditioning

Question 26Question →

A network engineer is evaluating QoS traffic conditioning strategies for a branch connection using a sub-rate Ethernet service where the committed information rate is lower than the physical interface speed. Which two characteristics accurately distinguish traffic policing from traffic shaping in this scenario? (Select two.)

Select all that apply

Traffic shaping buffers out-of-profile packets in memory queues to smooth egress traffic spikes.

Traffic policing immediately drops or remarks packets that exceed the configured bandwidth threshold without buffering.

Traffic policing utilizes interface queues to store bursty traffic, preventing packet loss on ingress links.

Traffic shaping discards excess frames as soon as the bandwidth limit is exceeded to prevent queuing delay.

Show answer & explanation

Answer: Traffic shaping buffers out-of-profile packets in memory queues to smooth egress traffic spikes.; Traffic policing immediately drops or remarks packets that exceed the configured bandwidth threshold without buffering.

Answer

Traffic shaping buffers out-of-profile packets in memory queues to smooth egress traffic spikes, and traffic policing immediately drops or remarks packets that exceed the configured bandwidth threshold without buffering.

Traffic shaping regulates output rates by queuing non-conforming packets and transmitting them smoothly over time. In contrast, traffic policing enforces rate limits without buffering, immediately dropping or remarking packets that exceed specified burst limits.

Step-by-Step Solution

Analyze operational behavior of traffic shaping

Shaping buffers excess bursts in queues and smooths the egress flow to match the configured rate, introducing queuing delay rather than immediate drops.

Shapers use queue buffers to manage micro-bursts and smooth out-of-profile traffic.

Analyze operational behavior of traffic policing

Policing checks traffic against rate limits and immediately drops or remarks non-conforming packets without using memory buffers.

Policers operate on a bucket mechanism with no queue storage capability.

Key Concept

Traffic Policing vs. Traffic Shaping Mechanics

Estimated Time:1m 30s

Question 27Question →

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?

At the Cisco IP Phone, trusting CoS markings generated by the phone while untrusting and remarking traffic from the connected PC to CoS 0.

At the access switch port, trusting all incoming CoS frames indiscriminately from both the IP Phone and the attached PC.

At the desktop PC network interface card (NIC), allowing the desktop operating system to mark data frames before transmission.

At the core switch interface, deferring all classification and trust boundary enforcement until traffic leaves the access tier.

Show answer & explanation

Answer: 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

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

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.

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

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

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