Priority Queuing – WAN Availability and QoS

By | 09/12/2024

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 because higher-priority queues must be emptied before lower-priority queues.

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, CQ proceeds to the next queue. The network operator can customize these byte size limits. CQ is fairer than PQ because it allows some level of service to all traffic. This queuing method is considered legacy due to improvements in the other queuing methods.

Weighted Fair Queuing

Weighted fair queuing (WFQ) ensures that traffic is separated into individual flows or sessions without requiring that you define ACLs. 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 by assigning bandwidth and specifying the 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 the key advantages of CBWFQ is its modular nature, which makes it extremely flexible for most situations. It is often referred to as Modular QoS CLI (MQC), which is the framework for building QoS policies. Many classes can be defined to separate network traffic as needed in the MQC.

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. Unlike PQ, LLQ provides for a maximum threshold on the priority queue to prevent lower-priority traffic from being starved by the priority queue.

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 for many VoIP networks.

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