Core Issue
The tx-ring-limit is automatically tuned only when the fair-queue command is configured on a interface. Therefore, if the no fair-queue command is configured on an interface, the serial interface hardware transmit buffer does not necessarily have the correct setting for the tx-ring-limit.
Also, when an individual link is part of a dialer group or multilink group, the link configured for the master interface is the one actually queued. If the fair-queue command is configured for the interface serial0/0, it automatically tunes its ring-size for low latency operation. At the same time, traffic is shaped as configured for the master multilink interface.
Resolution
To resolve this problem, issue the fair-queue command on the physical or virtual interface. On both routers (hub and spoke), add the fair-queue serial interface commands on all serial ports. The fair-queue command allows the prioritization to work correctly. Otherwise, the default buffer size is excessively large, and a large number of packets are placed within it. In this case, the Low Latency Queuing (LLQ) software does not sense normal congestion due to the serialization delay on the low speed serial interface.
The fair-queue command enables fair-queuing, and allows the LLQ to set the output hardware interface packet buffer (called the tx-ring) to a small value (generally less than 8-10 packets). If the buffer is set to the low value, only two packets are queued to the physical interface before "back pressure" is experienced in the LLQ operation.
Note: If the addition of the fair-queue command does not help, manually tune the tx-ring with the tx-ring-limit 2 serial interface command on the serial interfaces of both routers.
