350-501 Automation and Quality of Service Practice Question
A service provider is experiencing congestion on a core link connecting two P routers. The customer traffic is classified into three classes: voice (low latency), video (low loss), and data (best effort). The current configuration uses DSCP-based classification at the PE ingress, but many customers are remarking DSCP values to gain better service, bypassing the provider's QoS policy. The provider wants to enforce a strict trust boundary at the PE and re-mark all traffic according to a per-customer contract. Additionally, the provider must offer per-customer bandwidth guarantees, ensuring that one customer's traffic does not starve another customer's traffic on the congested core link. The solution must be scalable to hundreds of customers. What should the designer recommend?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Implement hierarchical QoS on the PE egress to customer-facing interfaces, with parent-level shaping per customer and child-level policing per class, and set a trust boundary to mark all traffic based on the customer contract at ingress.
Hierarchical QoS allows per-customer shaping at the parent level to enforce per-customer bandwidth limits, and per-class policing at the child level to enforce per-class contracts. This provides the required trust boundary and per-customer guarantees. Option A is wrong because a flat policy on core interfaces does not allow per-customer differentiation and cannot enforce per-customer guarantees. Option B is wrong because auto-qos does not provide per-customer granularity and relies on trust which is already broken. Option D is wrong because MPLS TE tunnels reserve bandwidth but do not solve the trust boundary issue; also resetting EXP is complex.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Apply a flat QoS policy on the core interfaces that polices each DSCP value to a fixed rate; trust is not needed because the core enforces its own limits.
Why it's wrong here
Flat policing on core cannot enforce per-customer contracts and does not prevent customer remarking from affecting other customers.
- ✗
Use auto-qos on all PE interfaces and rely on CoS trust; the core uses MPLS EXP derived from CoS to ensure proper queuing.
Why it's wrong here
Auto-qos does not offer per-customer traffic separation and trust is already compromised by customers remarking DSCP.
- ✓
Implement hierarchical QoS on the PE egress to customer-facing interfaces, with parent-level shaping per customer and child-level policing per class, and set a trust boundary to mark all traffic based on the customer contract at ingress.
Why this is correct
Hierarchical QoS provides both per-customer and per-class enforcement, and setting trust boundary at ingress solves the remarking issue.
- ✗
Configure MPLS Traffic Engineering tunnels on the core with bandwidth reservation per customer class; use EXP-null to preserve markings end-to-end.
Why it's wrong here
TE tunnels provide bandwidth but do not solve the trust boundary issue; remarking still requires trust boundary at PE.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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