Cisco IOS Priority Queuing Static Queue Structure

350-401 By islever Editorial Last updated

Interpret QoS configurations

Short answer

D Priority queuing uses four static queues classified as High, Medium, Normal, and Low, servicing them in strict priority order.

Question

Which congestion queuing method on Cisco IOS based routers uses four static queues?
  1. weighted fair
  2. custom
  3. low latency
  4. priority Correct Answer

Key Takeaways

The trap is confusing Custom Queuing's default four queues with Priority Queuing's strict four-level hierarchy; PQ is defined by its fixed high/medium/normal/low structure.

Priority queuing (PQ) is the Cisco IOS congestion management method that employs four static queues. Learn how PQ prioritizes traffic through strict hierarchy.

Common mistake: Candidates often select Custom Queuing (B) because it also defaults to four queues. However, Custom Queuing uses round-robin bandwidth sharing and supports up to 16 queues, whereas Priority Queuing is strictly defined by its four static priority levels.

Expert Analysis

Queue Structure and Mechanics

The question asks to identify a Cisco IOS congestion queuing method that utilizes exactly four static queues. To answer this, we must compare the architectural design of the four primary queuing algorithms available on legacy Cisco routers.

Priority Queuing (PQ) is defined by its strict hierarchical structure. It classifies traffic into precisely four predefined queues: High, Medium, Normal, and Low. The router services these queues in a strict priority order; it empties the High queue completely before moving to Medium, then Normal, and finally Low. This mechanism ensures that high-priority traffic is never delayed, but it introduces the risk of starvation for lower-priority classes if high-priority traffic is continuous. Because PQ relies on this fixed set of four buckets, it fits the "four static queues" description perfectly.

Custom Queuing (CQ), while often associated with the number four, functions differently. CQ uses a round-robin approach to service queues based on byte counts (bandwidth allocation). While the default configuration defines four queues, CQ allows administrators to configure up to sixteen queues. Furthermore, CQ is not strictly limited to "static" priority levels in the same way PQ is; it is bandwidth-weighted. Therefore, while it can use four queues, PQ is the method inherently defined by a rigid four-queue hierarchy.

Weighted Fair Queuing (WFQ) and Low Latency Queuing (LLQ) are dynamic or hybrid systems. WFQ calculates weights dynamically per flow and does not use a fixed set of static queues. LLQ is an enhancement to Class-Based Weighted Fair Queuing (CBWFQ) that adds a single strict priority queue to a system of weighted classes. Neither uses a simple, static four-bucket model.

Conclusion

The defining characteristic of Priority Queuing is its reliance on a fixed hierarchy of four distinct priority levels. Comments in the discussion highlight this exact behavior, noting that PQ places traffic into one of four queues based on priority level. Thus, PQ is the correct classification.

Official Reference

https://www.ciscopress.com/articles/article.asp?p=352991&seqNum=7

This link opens the official vendor documentation in a new tab.

Exam Strategy

When analyzing QoS questions, distinguish between 'strict priority' and 'weighted/bandwidth-based' methods. If a question mentions 'four static queues' and 'priority', think Priority Queuing (PQ). If it mentions 'round robin' or 'byte counts', think Custom Queuing (CQ). Remember that PQ can starve lower queues, a key behavioral trait often tested.

Frequently Asked Questions

What are the four queues in Priority Queuing?

The four queues are High, Medium, Normal, and Low. Traffic is processed from High to Low, ensuring high-priority packets are never delayed by lower-priority ones.

Does Custom Queuing use only four queues?

No. While Custom Queuing defaults to four queues, it can be configured to support up to sixteen queues. It uses a round-robin algorithm rather than strict priority.

Why might Priority Queuing cause problems?

Because PQ services higher queues completely before moving to lower ones, continuous high-priority traffic can starve medium, normal, and low-priority traffic, causing them to time out.

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Community Votes

D
60%
B
40%

60% of anonymous learners picked answer D. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 11 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 11

  1. Asymptote 👍 7 Answered D
    Priority Queuing (PQ)—This type of queuing places traffic into one of four queues. Each queue has a different level of priority, and higher-priority queues must be emptied before packets are emptied from lower-priority queues. This behavior can “starve out” lower- priority traffic. Reference: https://www.ciscopress.com/articles/article.asp?p=352991&seqNum=7#:~:text=Priority%20Queuing%20(PQ)%E2%80%94This%20type%20of%20queuing%20places%20traffic%20into%20one%20of%20four%20queues.%20Each%20queue%20has%20a%20different%20level%20of%20priority%2C%20and%20higher%2Dpriority%20queues%20must%20be%20emptied%20before%20packets%20are%20emptied%20from%20lower%2Dpriority%20queues.%20This%20behavior%20can%20%E2%80%9Cstarve%20out%E2%80%9D%20lower%2D%20priority%20traffic.
  2. Japsurd 👍 3 Answered D
    https://packetlife.net/media/library/19/QoS.pdf
  3. d656971 👍 2 Answered A
    "Which . . . uses four static queues?" Correct is A) Weighted Fair Queuing (also CBWFQ) B) maybe. . .if you're customizing the queueing sure, but there is usually a name tied with your queueing method. C) Same As PQ -- only 1, max 2 PQ or LLQ available, not four static queues. D) A PQ only uses ONE queue out of the four, except a few special platforms which have two priority queues (but they are not called static queues). I've never even heard of hardware that would allow all four queues to be "PQ". That would defeat the purpose of a PQ.
  4. slacker_at_work 👍 2
    Keyword is "four" queues: Priority Queuing. The Cisco CG-OS router supports four priority queue settings for QoS, indexes 1 to 4, where 1 is the highest priority and 4 is the lowest.
  5. Joseph123 👍 2
    Correctomundo
Show 6 more comments
  1. f4f09e6 👍 1 Answered D
    In Cisco networking, the queuing method that specifically uses four static queues is Priority Queuing (PQ). Key Characteristics of Priority Queuing (PQ) Queue Structure: It classifies traffic into exactly four predefined, static queues based on priority level: High, Medium, Normal, and Low.
  2. HaidariEshaq 👍 1 Answered B
    Custom Queuing (CQ) → Uses 4 static queues (actually up to 16 queues in total, but by default 4 are defined). Traffic is placed into queues based on defined policies, and packets are serviced in a round-robin fashion.
  3. Sam7007 👍 1 Answered B
    Custom Queuing = 4 queues with allocated bandwidth Priority Queuing = 4 queues with strict priority
  4. alexia_net 👍 1 Answered D
    Class-Based Weighted Fair Queuing (CBWFQ) with Low Latency Queuing (LLQ)
  5. straightAnswers 👍 1
    Thanks for sharing this
  6. alexia_net 👍 1
    Is priority, actually

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