Cisco IOS Routers: Low Latency Queuing Algorithm | Exam 400-051 CCIE Collaboration

Low Latency Queuing Algorithm

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In Cisco IOS routers that use low latency queuing, which algorithm is used to presort traffic going into the default queue?



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WFQ is a flow-based queuing algorithm used in Quality of Service (QoS) that does two things simultaneously: It schedules interactive traffic to the front of the queue to reduce response time, and it fairly shares the remaining bandwidth between high bandwidth flows.

A stream of packets within a single session of a single application is known as flow or conversation.

WFQ is a flow-based method that sends packets over the network and ensures packet transmission efficiency which is critical to the interactive traffic.

This method automatically stabilizes network congestion between individual packet transmission flows.

In Cisco IOS routers that use low latency queuing (LLQ), the algorithm used to presort traffic going into the default queue is the weighted fair queuing (WFQ) algorithm.

LLQ is a QoS mechanism that allows the router to prioritize delay-sensitive traffic, such as voice and video, while still providing fair access to other types of traffic. LLQ has four queues: three priority queues (high, medium, and low) and one default queue. The priority queues are serviced using strict priority queuing, while the default queue is serviced using WFQ.

WFQ is a queuing algorithm that provides fair access to the router's resources by assigning weights to each flow based on its packet size. When packets arrive at the router, they are classified into flows based on their source and destination addresses and port numbers. WFQ then assigns each flow a weight based on its packet size, with larger packets receiving a higher weight. The router then services packets from each flow in a round-robin fashion, with the weight of each flow determining its share of the bandwidth.

In LLQ, the default queue is used to hold traffic that is not classified into one of the priority queues. This can include data traffic, such as file transfers and web browsing, as well as non-real-time applications, such as email and instant messaging. Because this traffic is less time-sensitive than voice and video traffic, it is assigned a lower priority and serviced using WFQ to ensure that it does not interfere with the transmission of more critical traffic.

In conclusion, the algorithm used to presort traffic going into the default queue in Cisco IOS routers that use LLQ is the weighted fair queuing (WFQ) algorithm.