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ccna pdf free download.Congestion occurs when the traffic that a network device needs to forward is larger than the traffic that can be sent out of the interface. When congestion occurs, it is necessary to use queuing technology to queue packets to achieve priority scheduling and priority forwarding of important data flows, so queuing technology is one of the methods to solve congestion management. It should be emphasized that queuing technology is only applied to the outgoing direction of the interface of the network equipment, and only one queuing technology can be used for one interface. There is a software queue and a hardware queue on each interface of a network device. The packets that need to be forwarded out of the interface enter the software queue first, then the packets are scheduled using various queuing techniques, and then the packets enter the hardware queue, which always sends the packets out on a first-in-first-out basis. If the hardware queue is not full, it means that the outgoing interface can forward the packet in time, at which point there is no need to queue and it is forwarded directly.21.2.1 First-In-First-Out (FIFO) QueueingFIFO buffers and forwards data in the order of packet arrival; it has no concept of priority or traffic class, but only one queue, and treats all packets equally. Packets are forwarded in the order in which they arrive at the interface. By default, the router uses FIFO for scheduling when the speed of the interface is greater than 2.048 Mbps. FIFO queues have the advantage of simple processing and low overhead. However, FIFO does not distinguish packet types and uses best-effort forwarding mode, so that the delay of time-sensitive real-time applications (such as VoIP) is not service-guaranteed, and the bandwidth of critical services is not service-guaranteed.21.2.2 Priority Queueing (PQ)PriorityQueuing (PQ) is designed for business-critical applications. Critical services have an important feature that requires priority access to services when congestion occurs to reduce the response delay. There are four queues in PQ: high, medium, normal, and low priority. Packets are placed in different queues according to a prior classification, and routers schedule packets in the order of high, medium, normal, and low, and only when the high priority queue is empty will they schedule packets from the medium priority queue, and so on. This ensures that high priority packets must be forwarded first. However, if the high-priority queue is not empty for a long time, the low-priority queue will never be forwarded. A length can be set for each queue. the tail of the packet will be discarded when the queue is full.Chapter 21 QoS .419 .The advantage of PQ is that high priority packets are guaranteed to be forwarded. the disadvantage is that when there are always packets in the higher priority queues. the packets in the lower priority queues will remain unserved and the queue will "starve". so this queue is rarely invoked directly under the interface.21.2.3 Weighted Fair Queue (WFQ)WeightedFairQueuing (WFQ) classifies packets by flow characteristics. For IP networks, packets with the same source IP address, destination IP address, source port number, destination port number, protocol number, and ToS belong to the same flow, which is automatically calculated using the HASH algorithm. Each stream is assigned to a queue, and this approach tries to group streams with different characteristics into different queues. Each queue class can be considered as a class of streams whose packets enter the same queue in WFQ. The number of queues allowed by WFQ is limited and the user can configure this value as needed. When exiting the queue, WFQ allocates the egress bandwidth that each flow should occupy according to the flow's priority. The smaller the value of the priority, the smaller the bandwidth obtained. The larger the value of the priority, the larger the bandwidth obtained. This ensures fairness between services of the same priority and reflects the weighting between services of different priorities. Cisco routers use weighted fair queuing by default on interfaces when the bandwidth of the interface is less than 2.048 Mbps. The advantages of WFQ are that it is simple to configure, facilitates the forwarding of smaller packets, and each flow gets fair scheduling while taking care of the benefits of high-priority flows. However, it lacks some flexibility because streams are automatically classified and manual intervention is not possible, and it is limited by resources and cannot provide accurate service when multiple streams enter the same queue and cannot guarantee the actual bandwidth each stream receives. WFQ balances the delay and jitter of individual streams, and thus is not suitable for delay-sensitive service applications.21