Which QoS Class Model Fits CS5 Broadcast?
An ISP is going to offer premium broadcast services to its clients. Services will be marked with CS5 and will be using 10% of the link bandwidth. All links are at 10 Gbps in the backbone and 1 Gbps toward end clients with GPON technology. The ISP wants to use the minimum required resources on the network nodes and for it to be easily scalable. Which QoS strategy model must the ISP select to offer the premium service?
Community Insight
The test is matching the CS5 Broadcast Video DSCP to the smallest Cisco class model that natively contains it, and the trap is assuming that more classes (12-class) are needed whenever a premium service is involved.
This 300-420 QoS design question asks which Cisco QoS strategy model an ISP should deploy for a CS5-marked premium broadcast service while consuming the fewest node resources. The answer is the 8-class model, because CS5 (Broadcast Video) first appears in the 8-class hierarchy, and it avoids the extra per-class queues, policers and configuration of the 12-class model.
Option C (12-class) is the most tempting wrong pick, because commenters map CS5 to Broadcast Video inside the 12-class DSCP list; but 12-class exists to classify streaming, OAM, transactional and bulk data — none of which the ISP asked for — so it violates the explicit 'minimum required resources' constraint.
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Expert Analysis
Why the Answer Is Correct
The ISP's premium broadcast service is marked CS5, and in Cisco's standard QoS strategy models CS5 is assigned to Broadcast Video — a class that only exists from the 8-class model upward. The 8-class model contains Best Effort, Voice (EF), Broadcast Video (CS5), Real-Time Interactive (CS4), Multimedia Conferencing (AF41), Network Control (CS6), Signaling (CS3) and Scavenger (CS1), so it satisfies the marking requirement with the smallest class set. Because the requirement is explicitly minimum node resources plus easy scalability, the smallest model that natively carries CS5 is the right design choice. Bandwidth-wise the service is only 10% of a 10 Gbps backbone link (1 Gbps) or a 1 Gbps GPON uplink (100 Mbps), so a single dedicated broadcast queue in an 8-class hierarchy is entirely sufficient.Why the Other Options Are Wrong
Option B (4-class) and Option D (6-class) both fail the hard requirement: neither hierarchy reserves a CS5 class, so premium broadcast would have to be lumped into Best Effort, Critical Data (AF31) or the EF priority queue, either losing its guarantee or stealing the voice guarantee. Option C (12-class) does contain CS5, but it adds four further classes — Multimedia Streaming (AF31), Management/OAM (CS2), Transactional Data (AF21) and Bulk Data (AF11) — each demanding its own queue, bandwidth threshold, policer and policy-map entry on every 10 Gbps backbone and 1 Gbps GPON node. That is precisely the additional resource cost the ISP said it wanted to avoid, and it complicates rollout across many access nodes for a single service. Only the 8-class model (Option A) simultaneously delivers CS5 and keeps the per-node footprint minimal and easy to extend to future services.Community Comment Notes
MaxwellJK selected Option C while pasting the full twelve-entry DSCP mapping from Voice/EF down to Scavenger/CS1; that mapping itself is accurate, but it describes the largest model rather than the minimum one the ISP demanded. drinu89 chose Option A, calling the 8-class model "the middle ground" with "Separate classes for premium services/video traffic" and noting it is a "Common Cisco design recommendation when greater service differentiation is needed." drinu89's reasoning also stresses "Good scalability. Less complexity than 12-class," which matches the question's twin constraints of minimum resources and scalability, so Option A stands as the verified answer.Exam Strategy
Map the required DSCP value to its Cisco class name first (CS5 = Broadcast Video), then pick the smallest class model that contains that class — never the largest model that contains it. In 300-420 QoS questions, phrases like 'minimum required resources' and 'easily scalable' are code words for 'choose the lowest-tier model that still meets the marking requirement.'
Frequently Asked Questions
Why can't the ISP use the 4-class or 6-class model for a CS5 service?
Neither the 4-class nor the 6-class hierarchy reserves DSCP CS5, so the premium broadcast traffic would have to share Best Effort, AF31 or the EF priority queue and would lose its dedicated guarantee.
Why not choose the 12-class model since it also supports CS5?
The 12-class model adds streaming, OAM, transactional and bulk data classes, each needing extra queues, policers and policy configuration on every node, which breaks the ISP's requirement for minimum resource usage and simple scaling.
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