Congestion
Management and Avoidance
Congestion management, commonly called queuing, refers to how a router or switch manages
packets or frames while they wait to exit a device. With routers, the waiting occurs when IP
forwarding has been completed, so the queuing is always considered to be output queuing. LAN
switches often support both output queuing and input queuing, where input queuing is used for
received frames that are waiting to be switched to the switch’s output interfaces.
Congestion avoidance refers to the logic used when deciding if and when packets should be
dropped as a queuing system becomes more congested. This chapter covers a wide variety of
Cisco IOS queuing tools, along with the most pervasive congestion avoidance tool, namely
weighted random early detection (WRED).