300-420 Question #339: Real Exam Question with Answer & Explanation

Question

An engineer is designing a QoS solution for a campus. The design must guarantee real-time traffic delivery during congestion, minimize the bandwidth consumption for possible virus or worm attacks, and reduce flooding of excessive traffic during times of congestion. Which two solutions must the engineer select? (Choose two.)

Options

• ACreate a policing policy to drop excessive traffic and a strict queue for real-time traffic
• BApply queuing on the distribution to core links
• CCreate a scavenger queue for excessive traffic and a strict queue for real-time traffic
• DApply queuing on the access to distribution links
• ECreate a shaping policy to drop excessive traffic and a strict queue for real-time traffic

Explanation

To guarantee real-time traffic delivery, minimize attack impact, and reduce excessive flooding during congestion, the QoS design must include a strict priority queue for real-time traffic, a scavenger queue for excessive traffic, and apply queuing on the access to distribution links.

Common mistakes.

• A. While a strict queue for real-time traffic is good, using a policing policy to drop excessive traffic might be too aggressive, leading to TCP retransmissions and potentially worsening congestion or making applications unstable; a scavenger queue is generally preferred over aggressive dropping for non-critical traffic during congestion.
• B. While queuing can be applied anywhere, the most effective place to manage traffic and prevent congestion from propagating is at the network's edge or where oversubscription occurs, making queuing on access to distribution links a more critical initial placement than distribution to core links.
• E. Creating a shaping policy to drop excessive traffic is not the primary function of shaping; shaping buffers excessive traffic and smooths its transmission to conform to a rate, rather than dropping it directly for congestion management in the same way a scavenger queue would deprioritize it.

Concept tested. QoS mechanisms (queuing, strict priority, scavenger), congestion management, traffic prioritization

Reference. https://www.cisco.com/c/en/us/td/docs/solutions/Enterprise/Campus/campqos/qos_chap3.html

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