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.
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.