Which of the following does RED and WRED address? (Select the best answer.)
A. tail drop
B. bandwidth starvation
C. bandwidth guarantees
D. strict-priority queuing
Explanation:
Random early detection (RED) and weighted RED (WRED) are congestion avoidance mechanisms that address tail drop, which occurs when new incoming packets are dropped because a router’s queues are too full to accept them. Tail drop particularly affects Transmission Control Protocol (TCP) traffic, because when TCP packets are dropped, the sources of the traffic must retransmit the lost TCP packets. Additionally, the TCP traffic sources will detect the congestion and will correspondingly slow down the rate at which they send data until the congestion clears. When the congestion clears, the TCP sources speed up data transmission, which again causes congestion; this ebb and flow of traffic is called global TCP synchronization. RED mitigates the problems caused by global TCP synchronization by randomly dropping packets as congestion increases and before the queue becomes full. As the average size of the queue increases, RED will randomly drop packets at an increasingly faster rate. WRED improves upon RED by employing different tail drop thresholds for each IP precedence or Differentiated Services Code Point (DSCP) value, whereby lower-priority traffic is more likely to be dropped than higher-priority traffic.
RED and WRED do not address bandwidth starvation. Queuing methods, such as weighted fair queuing (WFQ), class based WFQ (CBWFQ), or low latency queuing (LLQ), mitigate bandwidth starvation. Bandwidth starvation occurs when higher-priority queues monopolize an interface’s bandwidth so that traffic from lower-priority queues is never sent.
RED and WRED do not address bandwidth guarantees. CBWFQ and LLQ provide bandwidth guarantees by allowing the creation of up to 64 custom traffic classes, each with a guaranteed minimum bandwidth. Bandwidth can be allocated as a value in Kbps, as a percentage of bandwidth, or as a percentage of the remaining bandwidth.
RED and WRED do not address strictpriority queuing. LLQ improves upon CBWFQ through the support of strictpriority queues that can be used for delay sensitive traffic. The strictpriority queues can use as much bandwidth as possible but can use only the guaranteed minimum bandwidth when other queues have traffic to send, thereby avoiding bandwidth starvation for the lowerpriority queues.
Reference:
https://www.cisco.com/c/en/us/td/docs/ios/12_2/qos/configuration/guide/fqos_c/qcfconav.html