Ticket#8 – VOIP Choppy Audio – DSCP Marking Mismatch Fixed [QoS Real World] [CCNP ENTERPRISE]

---

Problem Summary

Users across multiple floors reported choppy VoIP audio during internal calls. The problem was intermittent but noticeable during peak hours.

---

Symptoms Observed

• Audio drops every few seconds
• Jitter spikes over 40 ms
• MOS score degraded to <3.5
• Pings show minimal latency (<5ms) and no packet loss

---

Root Cause Analysis

Upon initial inspection, end-to-end ping and traceroute were normal. However, show policy-map interface revealed DSCP marking inconsistencies between access and distribution layers.

We found:

• IP Phones marked traffic as EF (DSCP 46)
• Access switches trusted CoS but didn’t reclassify DSCP properly
• Distribution switches were configured to match on DSCP — and thus weren’t queueing voice traffic correctly

---

The Fix

1. Enabled DSCP trust on access ports connected to IP phones: interface FastEthernet0/1 mls qos trust dscp
2. Verified marking using: show mls qos interface FastEthernet0/1 statistics
3. Matched DSCP EF in policy maps at distribution/core layer
4. Confirmed voice queue behavior using: show policy-map interface <interface-name>

---

EVE-NG Lab Topology

Configurations:

Access Switch:

mls qos
interface FastEthernet0/1
 mls qos trust dscp

Dist Switch:

class-map match-all VOICE
 match dscp ef

policy-map QOS-OUT
 class VOICE
  priority percent 30

interface Gi0/1
 service-policy output QOS-OUT

---

Verification

Command	Purpose	Example Output
show mls qos interface	Verifies DSCP/CoS markings	Displays DSCP EF stats
show policy-map interface	Checks QoS queueing	Shows voice priority queue
ping & traceroute	Basic L3 health	Stable latency, no loss

---

Key Takeaways

• DSCP markings must be preserved end-to-end for QoS to function properly.
• Don’t assume CoS/DSCP trust is active — explicitly configure it.
• Use policy maps that match your real traffic classes, not defaults.
• Monitoring tools (like MOS, jitter, and show policy-map) are essential.

---

Best Practice/Design Tips

• Always trust DSCP on IP phone ports, not on PCs
• Use LLDP-MED or CDP to detect IP phones and dynamically assign QoS
• Classify and prioritize voice using priority queues, not just bandwidth percent
• Avoid re-marking voice to lower DSCP values in the path

---

FAQs

Q1: What is DSCP and why is EF (DSCP 46) used for voice?
Answer:
DSCP (Differentiated Services Code Point) allows packet prioritization. EF (46) ensures voice packets are sent with highest priority, minimizing delay and jitter.

---

Q2: What’s the difference between trusting CoS and DSCP on access ports?
Answer:

• Trusting CoS reads Layer 2 markings (802.1p).
• Trusting DSCP honors Layer 3 IP packet markings.
  Best to trust DSCP if the phone sets markings.

---

Q3: Why was traffic not prioritized despite being marked as EF?
Answer:
Because downstream switches were matching on DSCP, but the access switch wasn’t properly classifying or trusting DSCP from phones.

---

Q4: How do I confirm DSCP is being trusted?
Answer:
Use:

show mls qos interface <int> statistics

It will show how many packets are marked per DSCP value.

---

Q5: How do I verify if voice traffic is being queued correctly?
Answer:
Use:

show policy-map interface <int>

Look under the class VOICE section for packets/bytes and drop stats.

---

Q6: Can DSCP markings be stripped or overwritten?
Answer:
Yes. If a switch has mls qos enabled but no trust on the interface, it resets DSCP to 0 by default.

---

Q7: What’s the difference between priority queue and bandwidth queue?
Answer:

• Priority queue (PQ): Low latency, strict priority (good for voice).
• Bandwidth queue: Assures minimum bandwidth but adds delay.

---

Q8: How do I simulate VoIP traffic in EVE-NG for testing?
Answer:
You can:

• Use traffic generators (like Spirent/IXIA)
• Or mark ICMP traffic with DSCP EF using:

ping <ip> dscp 46 repeat 100

---

Q9: What tools help with QoS troubleshooting?
Answer:

• Wireshark (for DSCP/CoS inspection)
• IP SLA (for MOS, jitter, delay)
• show policy-map, debug qos on routers/switches

---

Q10: How to handle DSCP values over WAN/MPLS circuits?
Answer:
Coordinate with the provider to preserve or remap DSCP values. Use QoS shapers and markers at the edge.

---

YouTube Link

Watch the Complete CCNP Enterprise: VOIP Choppy Audio – DSCP Marking Mismatch Fixed [QoS Real World] Demo & Explanation on our channel:

---

Final Note

Understanding how to differentiate and implement VOIP Choppy Audio – DSCP Marking Mismatch Fixed [QoS Real World] is critical for anyone pursuing CCNP Enterprise (ENCOR) certification or working in enterprise network roles. Use this guide in your practice labs, real-world projects, and interviews to show a solid grasp of architectural planning and CLI-level configuration skills.

If you found this article helpful and want to take your skills to the next level, I invite you to join my Instructor-Led Weekend Batch for:

CCNP Enterprise to CCIE Enterprise – Covering ENCOR, ENARSI, SD-WAN, and more!

Get hands-on labs, real-world projects, and industry-grade training that strengthens your Routing & Switching foundations while preparing you for advanced certifications and job roles.

Email: info@networkjourney.com
WhatsApp / Call: +91 97395 21088

Upskill now and future-proof your networking career!

---