/mcpQuality of Service for Voice over IP QoS Queueing Mechanisms 6 QoSVoIP.mif determine what should happen to each class. Then you apply the policy to incoming or outgoing traffic on an interface using the service-policy interface configuration command. The following configuration example shows how to use Modular QoS to classify and mark packets: QoS Queueing Mechanisms After all traffic has been placed into QoS classes based on their QoS requirements, you need to provide bandwidth guarantees and priority servicing through an intelligent output queueing mechanism. This section describes queueing mechanisms and includes the following subsections: • Low Latency Queueing • LLQ Configuration Example • Other QoS Queueing Mechanisms Low Latency Queueing A priority queue is required for VoIP. You can use any queueing mechanism that effectively gives VoIP high priority, but low latency queueing (LLQ) is recommended because it is flexible and easy to configure. The most flexible queueing method that satisfies VoIP requirements is LLQ. LLQ uses the MQC configuration method to provide priority to certain classes and to provide guaranteed minimum bandwidth for other classes. During periods of congestion, the priority queue is policed at the configured rate so that the priority traffic does not monopolize all the available bandwidth. (If the Configuration Example 4: Classification and Marking Using MQC access-list 100 permit udp any any range 16384 32767 access-list 100 permit tcp any any eq 1720 ! class-map voip match access-group 100 ! policy-map mqc class voip set ip precedence 5 <<#various other QoS commands>> class class-default set ip precedence 0 <<#various other QoS commands>> ! interface Ethernet0/0 service-policy input mqc In this example, any traffic that matches access list 100 will be classified as class voip and set with IP Precedence 5—meaning that the three most significant bits of the IP TOS byte are set to 101. Access list 100 here matches the common UDP ports used by VoIP and H.323 signaling traffic to TCP port 1720. All other traffic is set with IP Precedence 0. The policy is called mqc and is applied to incoming traffic on Ethernet interface 0/0.