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QoS is an effective tool for managing a WAN's available bandwidth. Keep in mind that QoS does not add bandwidth; it only helps you make better use of it. For chronic congestion problems, QoS is not the answer; you need to add more bandwidth. However, by prioritizing traffic, you can make sure that your most critical traffic gets the best treatment and available bandwidth in times of congestion. One popular QoS technique is to classify your traffic based on a protocol type or ACL and then give treatment to the class. You can define many classes to match or identify your most important traffic classes. The remaining unmatched traffic then uses a default class in which the traffic can be treated as best effort.
Queuing, Traffic Shaping, and Policing
Cisco has developed many different QoS mechanisms such as queuing, policing, and traffic shaping to enable network operators to manage and prioritize the traffic flowing on the network. Applications that are delay-sensitive require special treatment to avoid dissatisfaction by the user community, such as Voice over X technologies. Two types of output queues are available on routers—the hardware queue and the software queue. The hardware queue uses the strategy of first in, first out (FIFO). The software queue schedules packets first and then places them in the hardware queue. Keep in mind that the software queue is used only during periods of congestion. The software queue uses QoS techniques such as Priority Queuing, Custom Queuing, Weighted Fair Queuing, Class-Based Weighted Fair Queuing, Low-Latency Queuing, and traffic shaping and policing.
Priority Queuing
Priority Queuing (PQ) is a queuing method that establishes four interface output queues that serve different priority levels—high, medium, default, and low. Unfortunately, PQ can starve other queues if too much data is in one queue.
Custom Queuing
Custom Queuing (CQ) uses up to 16 individual output queues. Byte size limits are assigned to each queue so that when the limit is reached, it proceeds to the next queue. The network operator can customize these byte size limits. CQ is more fair than PQ because it allows some level of service to all traffic. This queuing method is considering legacy due to the improvements in the queuing methods.
Weighted Fair Queuing
Weighted Fair Queuing (WFQ) ensures that traffic is separated into individual flows or sessions without requiring that you define access control lists (ACL). WFQ uses two categories to group sessions—high bandwidth and low bandwidth. Low-bandwidth traffic has priority over high-bandwidth traffic. High-bandwidth traffic shares the service according to assigned weight values. WFQ is the default QoS mechanism on interfaces below 2.0 Mbps.
Class-Based Weighted Fair Queuing
Class-Based Weighted Fair Queuing (CBWFQ) extends WFQ capabilities by providing support for modular user-defined traffic classes. CBWFQ lets you define traffic classes that correspond to match criteria, including ACLs, protocols, and input interfaces. Traffic that matches the class criteria belongs to that specific class. Each class has a defined queue that corresponds to an output interface.
After traffic has been matched and belongs to a specific class, you can modify its characteristics, such as assigning bandwidth, maximum queue limit, and weight. During periods of congestion, the bandwidth assigned to the class is the guaranteed bandwidth that is delivered to the class.
One of CBWFQ's key advantages is its modular nature, which makes it extremely flexible for most situations. Many classes can be defined to separate your network traffic as needed. CBWFQ is becoming the "standard QoS mechanism" for networks that are not using VoIP.
Low-Latency Queuing
Low-Latency Queuing (LLQ) adds a strict priority queue to CBWFQ. The strict priority queue allows delay-sensitive traffic such as voice to be sent first, before other queues are serviced. That gives voice preferential treatment over the other traffic types.
Without LLQ, CBWFQ would not have a priority queue for real-time traffic. The additional classification of other traffic classes is done using the same CBWFQ techniques. LLQ is the standard QoS method of choice for Voice over IP networks.
Traffic Shaping and Policing
Traffic shaping and policing are mechanisms that take an action based on the traffic's characteristics, such as DSCP or IP precedence bits set in the IP header.
Traffic shaping slows down the rate at which packets are sent out an interface by matching certain criteria. Traffic shaping uses a token bucket technique to release the packets into the output queue at a preconfigured rate. Traffic shaping helps eliminate potential bottlenecks by throttling back the traffic rate at the source.
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