Answer
Random Early Discard (RED), also known as Random Early Detection, is a congestion avoidance mechanism. It helps to mitigate the impact of congestion (specifically with TCP-based traffic).When the number of packets queued is greater than the ability of the device to empty a queue, the queue requires a method for determining which packets to drop from the network. To address this, the Junos OS provides the option of enabling RED on individual queues.
Loss priority settings help determine which packets are dropped from the network during periods of congestion. The software supports multiple packet loss priority (PLP) designations: low and high. (In addition, medium-low and medium-high PLPs are supported when you configure tricolor marking.) You can set PLP by configuring a behavior aggregate or multifield classifier.
You can configure drop profiles that determine the probability of dropping a queued packet based on the fullness of the queue and then map those drop profiles to PLPs for a particular scheduler. For example, you would create a drop profile that drops packets aggressively and map it to the high PLP and map a conservative drop profile to the low PLP.
For more information on managing congestion through the use of RED drop profiles and packet loss priorities, see https://www.juniper.net/techpubs/en_US/junos15.1/topics/concept/red-drop-profile-overview-cos-config-guide.html.