/mcpQuality of Service for Voice over IP QoS Queueing Mechanisms 7 QoSVoIP.mif priority traffic monopolizes the bandwidth, it prevents bandwidth guarantees for other classes from being met.) If you provision LLQ correctly, the traffic going into the priority queue should never exceed the configured rate. LLQ also allows queue depths to be specified to determine when the router should drop packets if too many packets are waiting in any particular class queue. There is also a class default that is used to determine treatment of all traffic not classified by a configured class. The class default can be configured with the fair-queue interface configuration command, which means that each unclassified flow will be given an approximately equal share of the remaining bandwidth. Figure 1 shows how LLQ works. Figure 1 LLQ Operation In Figure 1, all traffic going out of an interface or subinterface (for Frame Relay and ATM) is first classified using MQC. There are four classes: one high priority class, two guaranteed bandwidth classes, and a default class. The priority class traffic is placed into a priority queue and the guaranteed bandwidth class traffic is placed into reserved queues. The default class traffic can be given a reserved queue or can be placed in an unreserved default queue where each flow will get an approximately equal share of the unreserved and available bandwidth. The scheduler services the queues so that the priority queue traffic is output first unless it exceeds a configured priority bandwidth and this bandwidth is needed by a reserved queue (that is, there is congestion). The reserved queues are serviced according to their reserved bandwidth, which the scheduler uses to calculate a weight. The weight is used to determine how often a reserved queue is serviced and how many bytes are serviced at a time. The scheduler services are based on the weighted fair queueing (WFQ) algorithm, a discussion of which is beyond the scope of this document. If the priority queue fills up because the transmission rate of priority traffic is higher than the configured priority bandwidth, the packets at the end of the priority queue will be dropped only if no more unreserved bandwidth is available. None of the reserved queues are restricted to the configured bandwidth if bandwidth is available. Packets violating the guaranteed bandwidth and priority are dropped only during periods of congestion. You must therefore provision the priority queue with enough bandwidth to handle all the VoIP traffic requiring priority servicing. 60595 Classification Class priority Priority queue Class 1 Reserved queue Class 2 Reserved queue Class default Reserved queue or unreserved default queue Scheduler Output