6.2.b Implement, optimize and troubleshoot QoS using MQC

6.2.b [v] Congestion management [queuing]

Congestion management features allow you to control congestion by determining the order in which packets are sent out an interface based on priorities assigned to those packets. Congestion management entails the creation of queues, assignment of packets to those queues based on the classification of the packet, and scheduling of the packets in a queue for transmission. The congestion management QoS feature offers four types of queuing protocols , each of which allows you to specify creation of a different number of queues, affording greater or lesser degrees of differentiation of traffic, and to specify the order in which that traffic is sent.

During periods with light traffic, that is, when no congestion exists, packets are sent out the interface as soon as they arrive. During periods of transmit congestion at the outgoing interface, packets arrive faster than the interface can send them. If you use congestion management features, packets accumulating at an interface are queued until the interface is free to send them; they are then scheduled for transmission according to their assigned priority and the queuing mechanism configured for the interface. The router determines the order of packet transmission by controlling which packets are placed in which queue and how queues are serviced with respect to each other.

There four types of queuing, which constitute the congestion management QoS features:

● FIFO (first-in, first-out). FIFO entails no concept of priority or classes of traffic. With FIFO , transmission of packets out the interface occurs in the order the packets arrive.

● Weighted fair queuing (WFQ). WFQ offers dynamic, fair queuing that divides bandwidth across queues of traffic based on weights. (WFQ ensures that all traffic is treated fairly, given its weight.) To understand how WFQ works, consider the queue for a series of File Transfer Protocol (FTP) packets as a queue for the collective and the queue for discrete interactive traffic packets as a queue for the individual. Given the weight of the queues, WFQ ensures that for all FTP packets sent as a collective an equal number of individual interactive traffic packets are sent.)

Given this handling, WFQ ensures satisfactory response time to critical applications, such as interactive, transaction-based applications, that are intolerant of performance degradation. For serial interfaces at E1 (2.048 Mbps) and below, flow-based WFQ is used by default. When no other queuing strategies are configured, all other interfaces use FIFO by default.

There are four types of WFQ:

● Flow-based WFQ (WFQ)

● Distributed WFQ (DWFQ)

● Class-based WFQ (CBWFQ)

● Distributed class-based WFQ (DCBWFQ)

● Custom queuing (CQ). With CQ, bandwidth is allocated proportionally for each different class of traffic. CQ allows you to specify the number of bytes or packets to be drawn from the queue, which is especially useful on slow interfaces.

● Priority queuing (PQ). With PQ, packets belonging to one priority class of traffic are sent before all lower priority traffic to ensure timely delivery of those packets.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 5719-5730).  . Kindle Edition.

http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/qos_conmgt/configuration/15-mt/qos-conmgt-15-mt-book/qos-conmgt-oview.html