A network administrator configures Low Latency Queuing (LLQ) on a Cisco WAN router egress … — Cisco CCNA

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.Answer
B
The router buffers the excess $72\text{ kbps}$ of voice traffic in RAM until link utilization drops below capacity.
C
The router dynamically reallocates unused bandwidth from the AF31 queue to transmit all $200\text{ kbps}$ of voice traffic without packet loss.
D
The router automatically demotes the DSCP marking of all voice packets from EF ( $46$ ) to AF11 ( $10$ ) and places them into the default queue.

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

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