350-501 Automation and Quality of Service Practice Question
A service provider has deployed a new MPLS L3VPN service for a customer with multiple sites. The customer reports intermittent voice quality issues during peak hours. The provider uses a DiffServ QoS model with MPLS EXP markings. The PE routers apply the following policy on the customer-facing interfaces:
policy-map CUSTOMER-OUT
class VOICE
priority percent 10 set mpls experimental topmost 5
class VIDEO
bandwidth remaining percent 30 set mpls experimental topmost 4
class DATA
bandwidth remaining percent 70 set mpls experimental topmost 0
The core network has a simple policy that maps EXP 5 to PQ, EXP 4 to a low-latency queue, and EXP 0 to best effort. The provider monitors the network and finds that the PE-CE interfaces are not congested (average utilization is 40%), but the voice packets are experiencing jitter. The customer's voice traffic is about 5% of the link capacity. Which action is most likely to reduce the jitter?
⚠ Common exam trap
Cisco often tests the misconception that simply increasing priority percentage or changing marking will solve jitter, when the real issue is the lack of explicit policing on the priority queue to match the actual traffic rate.
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
✓
Add a police command under the VOICE class to limit voice traffic to 5% and shape the priority queue.
The voice traffic is only 5% of link capacity, but the priority percent 10 command allows up to 10% of the link to be treated as priority. During peak hours, if other traffic (e.g., video or data) bursts and exceeds the remaining bandwidth, the priority queue can still be policed implicitly by the scheduler, but jitter arises because the priority queue is not rate-limited. Adding a police command under the VOICE class to limit voice to 5% and shaping the priority queue ensures that voice traffic does not exceed its actual rate, preventing microbursts that cause jitter in the priority queue.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Add a police command under the VOICE class to limit voice traffic to 5% and shape the priority queue.
Why this is correct
Policing and shaping the priority queue prevents bursts and reduces jitter.
- ✗
Configure the core routers to map EXP 5 to a low latency queue instead of PQ.
Why it's wrong here
Low latency queue is for video; voice should remain in PQ; the issue is at the PE-CE, not core.
- ✗
Increase the priority percent to 20 for voice traffic.
Why it's wrong here
Increasing priority does not solve jitter; it might allow voice to consume more bandwidth but does not shape it.
- ✗
Change the set mpls experimental topmost to set ip dscp ef for voice.
Why it's wrong here
DSCP marking is not used in the MPLS core; it does not help jitter.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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