TAILIEUCHUNG - Congestion Avoidance

When an interface on a router cannot transmit a packet immediately, the packet is queued, either in an interface Tx ring, or the interface output hold queue, depending on the switching path used. Packets are then taken out of the queue and eventually transmitted on the interface. If the arrival rate of packets to the output interface exceeds the router’s capability to buffer and forward traffic, the queues increase to their maximum length and the interface becomes congested. Tail drop is the router’s default queuing response to congestion. When the output queue is full and tail drop is in effect, all packets trying to enter (at the tail. | 5 Congestion Avoidance Overview This module describes the problems of congested networks. It introduces Random Early Detection RED WRED and Flow-based WRED as mechanisms to prevent congestion on router interfaces. Objectives Upon completion of this module you will be able to perform the following tasks Describe Random Early Detection RED Describe and configure Weighted Random Early Detection WRED Describe and configure Flow-based WRED Random Early Detection Overview The section describes the need for congestion avoidance in nearly-congested networks and explains the benefits of using RED on congested links. Objectives Upon completion of this lesson you will be able to perform the following tasks Explain the need for congestion avoidance mechanisms. Explain how RED works and how it can prevent congestion. Describe the benefits and drawbacks of RED. 5-2 Congestion Avoidance Copyright 2001 Cisco Systems Inc. Router Interface Congestion Router interfaces congest when the output queue is full - Additional incoming packets are dropped - Dropped packets may cause significant application performance degradation - By default routers perform tail-drop - Tail-drop has significant drawbacks - WFQ if configured has a more intelligent dropping scheme 2001 Cisco Systems Inc. Congestion Avoidance When an interface on a router cannot transmit a packet immediately the packet is queued either in an interface Tx ring or the interface output hold queue depending on the switching path used. Packets are then taken out of the queue and eventually transmitted on the interface. If the arrival rate of packets to the output interface exceeds the router s capability to buffer and forward traffic the queues increase to their maximum length and the interface becomes congested. Tail drop is the router s default queuing response to congestion. When the output queue is full and tail drop is in effect all packets trying to enter at the tail of the queue are dropped until the congestion is eliminated

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