350-801 · Question #379
How does WRED prevent congestion?
The correct answer is B. It drops packets selectively based on the IP precedence value. WRED prevents network congestion by selectively dropping packets based on their IP precedence or DSCP values when queue thresholds are exceeded. It prioritizes the retention of higher-priority traffic by dropping lower-priority traffic more aggressively.
Question
How does WRED prevent congestion?
Options
- AIt marks which packets to drop randomly.
- BIt drops packets selectively based on the IP precedence value.
- CIt marks which packets to drop based on how much traffic the client sent.
- DIt drops packets that have a higher DSCP value last.
How the community answered
(63 responses)- A5% (3)
- B92% (58)
- D3% (2)
Why each option
WRED prevents network congestion by selectively dropping packets based on their IP precedence or DSCP values when queue thresholds are exceeded. It prioritizes the retention of higher-priority traffic by dropping lower-priority traffic more aggressively.
WRED employs weighted randomness for dropping, meaning the drop probability increases with queue depth, and differently for various precedence/DSCP values, rather than just marking packets randomly without specific criteria.
WRED's dropping decisions are based on the queue depth and the packet's assigned QoS classification (e.g., IP precedence, DSCP), not on the volume of traffic sent by an individual client.
WRED is designed to protect higher-priority traffic; therefore, it drops packets with lower DSCP values first or more aggressively, ensuring that packets with higher DSCP values are dropped last (least likely to be dropped).
Concept tested: WRED congestion avoidance mechanism
Source: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/qos_conavd/configuration/xe-3s/qos-conavd-xe-3s-book/qos-conavd-wred.html
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