Implementing QoS mechanisms is another key consideration in order to ensure that VoIP traffic is forwarded across a network in a timely manner. A variety of different queuing mechanisms can be used on WAN interfaces to help prioritize voice traffic in order to ensure that it is serviced in this manner, and not delayed by other traffic that is less time-sensitive. While the four main queuing techniques typically implemented on Cisco router serial interfaces were looked at earlier in this chapter, voice traffic is typically prioritized using one of the three queuing methods listed below.
- Class-Based Weighted Fair Queuing (CBWFQ). Class-based WFQ works in a manner somewhat similar to traditional WFQ, with the exception that “classified” traffic can be placed into reserved bandwidth queues, ensuring that certain types of traffic (such as VoIP) are allocated a guaranteed amount of bandwidth. A scheduler services the queues based on the bandwidth assigned to them, also known as the “weight”. While CBWFQ ensures that all packets are allocated appropriate bandwidth based on their weight (and that all queues are serviced), it does not implement strict priority. In other words, this queuing method can still result in delays for VoIP traffic.
- Low Latency Queuing (LLQ). The LLQ queuing method is strongly recommended as the queuing method for use on WAN links that need to support time-sensitive traffic like VoIP. While LLQ functions in a manner very similar to CBWFQ, it does implement one very important additional feature, namely a priority queue. The priority queue is allocated a defined amount of priority bandwidth (weight), and is always serviced first as long as it does not exceed this bandwidth. Other types of traffic can be assigned to reserved queues (or a default queue) with pre-defined weights, ensuring that they are not starved of bandwidth.
- IP RTP Priority. The IP RTP Priority queuing method presents one of the simplest methods to ensure that VoIP packets are serviced with appropriate priority. When this queuing method is implemented, RTP voice packets (only) are automatically placed into a priority queue, while all other traffic is queued according to WFQ methods. IP RTP Priority can be implemented with a single command, which makes it an easy way to prioritize voice traffic, especially in environments where all other traffic can be handled equally. IP RTP priority does not become active until a WAN interface is experiencing congestion.
LLQ and IP RTP Priority are the two most popular queuing methods for prioritizing VoIP traffic.