Cisco CRS Command Reference Manual page 83

Quality of Service Commands on the Cisco IOS XR Software
Usage Guidelines
To use this command, you must be in a user group associated with a task group that includes appropriate task
IDs. If the user group assignment is preventing you from using a command, contact your AAA administrator
for assistance.
The RED congestion avoidance technique takes advantage of the congestion control mechanism of TCP. By
randomly dropping packets before periods of high congestion, RED tells the packet source to decrease its
transmission rate. Assuming the packet source is using TCP, it decreases its transmission rate until all the
packets reach their destination, indicating that the congestion is cleared. You can use RED as a way to cause
TCP to slow transmission of packets. TCP not only pauses, but it also restarts quickly and adapts its transmission
rate to the rate that the network can support.
RED distributes losses in time and maintains normally low queue depth while absorbing traffic bursts. When
enabled on an interface, RED begins dropping packets when congestion occurs at a rate you select during
configuration.
When time units are used, the guaranteed service rate is used to compute thresholds.
The default values for RED are calculated as follows:
default min_threshold = 0.03 * B
default max_threshold = 0.1 * B
where B is the bandwidth for the queue. When all the queues are congested, the bandwidth for the queue is
equal to the guaranteed service rate of the queue.
The mark probability is always set to 1.
When the value of the units argument is packets, packets are assumed to be 256 bytes in size.
Weighted Random Early Detection
The following restrictions apply to Weighted Random Early Detection (WRED):
• For thresholds in time units, the guaranteed service rate is used to calculate the thresholds in bytes.
• Default RED minimum threshold—30 ms at the guaranteed service rate.
• Default RED maximum threshold—100 ms as the guaranteed service rate.
For bundles, queue limit and WRED thresholds are supported in time units only.
RED is enabled when you configure any of the supported random-detect commands.
Note
The exponential weight factor is set to 0 for Cisco ASR 9000 series routers.
Note
Task ID
Task ID
qos
OL-23235-03
Cisco IOS XR Modular Quality of Service Command Reference for the Cisco CRS Router, Release 4.0
Operations
read, write
random-detect
77