Priority queuing

As explained in Acing the CCNA Exam: Volume 2 Advanced Networking and Security

PQ is a quality-of-service queuing technique that gives one queue strict priority over other queues.

Priority Queuing (PQ) is a QoS scheduling technique for congested interfaces. It separates classified packets into up to four ranked queues and always services the highest-priority nonempty queue first. section 10.2.3 section 10.4.1

PQ operates one packet at a time:

• Classify arriving packets and place them into separate priority queues. section 10.4.1
• Scan queues from highest to lowest priority before each transmission. section 10.4.1
• Transmit from a lower-priority queue only when every higher-priority queue is empty. section 10.4.1
• Recognize queue starvation: sustained higher-priority traffic can cause lower-priority queues to be serviced very infrequently. section 10.4.1

Figure 10.18 Priority Queuing (PQ). Packets are classified based on markings (the example markings in the figure are arbitrary) and added to a queue. The scheduler only services a lower-priority queue if all higher-priority queues are empty.

Visualizes classification by packet markings and the strict rule that lower-priority queues wait until all higher-priority queues are empty. section 10.4.1

How Acing the CCNA Exam: Volume 2 Advanced Networking and Security covers it

How deep it goes

Defines four ranked queues and packet-by-packet strict scheduling, then stops short of configuration and command details.

Examples

Uses a packet-marking diagram showing classification into queues and strict priority service.

What you can do after

Evaluate PQ behavior, priority order, and starvation risk in congested-interface scenarios.

How it connects

Contrasts PQ with FIFO, CBWFQ, and LLQ while connecting it to queuing, scheduling, classification, and QoS.

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Related concepts

• low latency queuingused by · Low Latency Queuing is a QoS scheduling technique that gives selected traffic strict priority treatment while providing bandwidth allocation for other classes.
• quality of serviceused by · Quality of service is the management of communication policies and traffic characteristics such as priority, reliability, latency, bandwidth, jitter, and loss.
• queuehas part · A queue is an ordered collection that holds items for retrieval or processing, typically removing them in the order they were added.
• queue starvationproduces · Queue starvation is the condition in which a lower-priority queue receives service infrequently or not at all because higher-priority queues remain active.
• priority queueuses · A priority queue is an abstract data type that retrieves or removes elements according to assigned priority rather than insertion order.

This explainer was generated by Manning's knowledge graph from the text of Acing the CCNA Exam: Volume 2 Advanced Networking and Security. The section links point to the parts of the book it draws on.