Campus QoS Design: Scavenger Queue and Queuing Placement?
Short answer
CD Create a scavenger queue for excessive traffic with a strict priority queue for real-time traffic, and apply queuing on access-to-distribution links.
Question
Key Takeaways
The question tests both QoS component selection (scavenger class + strict priority queue) and the placement rule of queuing at the access edge — the trap is picking policing/shaping (A or E) or pushing queuing up to the distribution-to-core links (B).
This campus QoS design question asks how to protect real-time traffic during congestion while containing virus/worm floods and excessive traffic. The correct pair is a scavenger-class queue plus a strict priority queue for real-time traffic (C), combined with applying queuing on the access-to-distribution links (D).
Common mistake: Learners most often pick A (policing to drop excessive traffic) alongside a correct scavenger queue, or choose B for placement. Policing hard-drops legitimate bursts instead of deferring them, and distribution-to-core links are intentionally overprovisioned (1:N收敛), so queuing belongs at the access edge where traffic sources attach.
Expert Analysis
Why the Answer Is Correct
Option C is the canonical Cisco Scavenger-class design: abnormal, worm, virus, or DoS-like traffic is marked Scavenger (CS1) and given a low-priority scavenger queue that is dropped most aggressively when congestion occurs, while real-time voice/video is placed in a strict-priority (LLQ) queue that is guaranteed bandwidth first. Option D is correct because queuing must be applied as close to the source of congestion as possible — at the access-to-distribution links, where end hosts and their traffic actually originate. This lets the campus design guarantee real-time delivery, shrink the impact of attack traffic, and reduce flooding during congestion, exactly the three goals stated.
Why the Other Options Are Wrong
Option A pairs policing with a correct strict queue, but policing as the primary congestion mechanism drops traffic abruptly and can harm legitimate TCP and bursty applications; the design intent here is marking to Scavenger plus queuing-based dropping during congestion. Option E is doubly wrong because traffic shaping buffers rather than drops excess traffic, so a "shaping policy to drop" is internally inconsistent and cannot meet the mitigation goal. Option B puts queuing on distribution-to-core links, which are designed with overprovisioned bandwidth ratios so that queuing is unnecessary and ineffective there; doing so also fails to protect traffic before it aggregates upward.
Community Comment Notes
As dougj explained, "you apply queueing as close as possible to source so it is between access and distribution," and he added that the design rule makes distribution-to-core bandwidth greater than access-to-distribution, which supports rejecting option B. drinu89 likewise noted that Cisco's Scavenger-class design couples campus access-edge policing/marking policies with Scavenger-class queuing for DoS/worm mitigation. MaxwellJK framed it as "Worm/undesirable traffic → Scavenger" with real-time guaranteed by a strict priority queue, agreeing on the C half even though he paired it with policing; the placement reasoning still makes D the second required choice.
Exam Strategy
When a campus QoS question mentions virus/worm, DoS, or excessive traffic, immediately look for the Scavenger class paired with a strict priority queue for real-time traffic. Then confirm placement: queuing belongs on access-edge (access-to-distribution) links, since distribution-to-core is overprovisioned by design.
Frequently Asked Questions
Why is a scavenger queue chosen instead of a policing policy for worm traffic?
Why apply queuing on access-to-distribution links rather than distribution-to-core?
Discussion 4 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 4
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dougj 👍 2Answer is not B, you apply queueing as close as possible to source so it is between access and distribution. You also have the design rule whereby distribution to core bandwidth should be greater than the access to distribution layers
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LearnMachine 👍 2B. Apply queuing on the distribution to core links C. Create a scavenger queue for excessive traffic and a strict queue for real-time traffic
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drinu89 👍 1Virus/worm/excessive traffic → Cisco recommends Scavenger-class QoS for DoS/worm mitigation. Cisco’s Scavenger strategy says abnormal/excess traffic is marked to Scavenger and then dropped most aggressively during congestion. This is the Cisco campus QoS design piece. Cisco’s Scavenger-class design specifically says that Campus Access-Edge policing policies are coupled with Scavenger-class queuing policies on the uplinks to the Campus Distribution Layer. That maps directly to access-to-distribution links.
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MaxwellJK 👍 1Worm/undesirable traffic → Scavenger Excessive traffic → Policing Real-time guarantee → Strict Priority Queue
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