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QoS configured on fastethernet interface not recognised and results in "choppy voice" even when the voice packets are matched against QoS

Core Issue

In a multi-site Cisco CallManager setup connected through an Ethernet WAN link, there are voice quality issues even with a Quality of Service (QoS) policy in place that prioritizes voice across the WAN.

Voice quality degrades intermittently to both the central site and out to the Public Switched Telephone Network (PSTN), even though Expedited Forwarding (EF) frames and data packets on both sites are being tagged correctly. Also, downloading the jitter jumps from an average of 6-8 up to an average of 40-50.

Resolution

The high-speed interface has a larger hardware buffer, which does not allow software queuing to detect the congestion and prioritize the traffic normally.

In order to resolve this issue, configure the tx-ring-limit to 2 to change the hardware buffer. This way, the Low Latency Queuing (LLQ) can prioritize the traffic normally.

Problem Type

Voice quality problems (Audio quality: Choppy voice, hissing, echo, etc.)

Voice Quality

Voice distortion (Read about voice distortion symptoms.)

Choppy voice (Listen to short or long choppy voice samples.)

Failure Type

Bad operational behavior

Nothing detected: Not detected, recorded, understood, recognized, etc.

Jitter

Comments
New Member

What Catalyst platforms does this refer to? In particular:

Cat 4500?

Cat 6500?

Cat35xx?

Cat29xx?

Cat3750?

Thanks

Jed

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