Exam 350-801 topic 1 question 338 discussion

Weighted random early detection (WRED) is a Cisco-proprietary congestion avoidance technique. WRED increases the probability that congestion (that is, an output interface queue completely filling to capacity) is avoided by dropping low-priority packets rather than dropping high-priority packets. selectively drop packets when buffers are filling up. This random drop is used to avoid congestion. "Implementing Cisco Unified Communications Voice over IP and QoS (CVOICE) Foundation Learning Guide"

B is correct

Answer B About Weighted Random Early Detection Random Early Detection (RED) is a congestion avoidance mechanism that takes advantage of the congestion control mechanism of TCP. By randomly dropping packets prior to periods of high congestion, RED tells the packet source to decrease its transmission rate. WRED drops packets selectively based on IP precedence. Edge routers assign IP precedences to packets as they enter the network. (WRED is useful on any output interface where you expect to have congestion. However, WRED is usually used in the core routers of a network, rather than at the edge.) WRED uses these precedences to determine how it treats different types of traffic. https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/qos_conavd/configuration/xe-16/qos-conavd-xe-16-book/qos-conavd-cfg-wred.html

In fact, RED (random early detection) randomly drops packets. WRED (weighted random early detection) literally selectively discards lower priority traffic. With WRED, IP Precedence governs which packets are dropped. So the answer 100% has to be "B".

A isn't the correct answer. The word "random" is intended to trick you into selecting it. While it's true, packets are randomly dropped prior to periods of high congestion, it actually uses IP Precedence to drop them. It even goes as far as using what's called drop probability, which is based on minimum threshold, maximum threshold, and mark probability denominator.

Answer B Answer A could not fit, it poorly explains on which decision/based on it marks traffic WRED uses these precedences to determine how it treats different types of traffic. https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/qos_conavd/configuration/15-mt/qos-conavd-15-mt-book/qos-conavd-cfg-wred.html

A is a correct answer https://support.huawei.com/enterprise/en/doc/EDOC1100125515/2f0b7510/introduction-to-congestion-management-and-congestion-avoidance#:~:text=WRED%20is%20a%20congestion%20avoidance,a%20burst%20of%20TCP%20retransmission.

https://www.cisco.com/c/en/us/td/docs/ios/qos/configuration/guide/12_2sr/qos_12_2sr_book/congestion_avoidance.html