Users Guide

Congestion avoidance
Congestion avoidance anticipates and takes necessary actions to avoid congestion. The following mechanisms avoid congestion:
● Tail drop—Packets are buffered at traffic queues. When the buffers are exhausted or reach the configured threshold,
excess packets drop. By default, OS10 uses tail drop for congestion avoidance.
● Random early detection (RED)—In tail drop, different flows are not considered in buffer utilization. When multiple hosts
start retransmission, tail drop causes TCP global re-synchronization. Instead of waiting for the queue to get filled up
completely, RED starts dropping excess packets with a certain drop-probability when the average queue length exceeds the
configured minimum threshold. The early drop ensures that only some of TCP sources slow down, which avoids global TCP
re-synchronization.
● Weighted random early detection (WRED)—This allows different drop-probabilities and thresholds for each color — red,
yellow, green — of traffic. You can configure the drop characteristics for three different flows by assigning the colors to the
flow. Assign colors to a particular flow or traffic using various methods, such as ingress policing, qos input policy-maps, and
so on.
● Explicit congestion notification (ECN)—This is an extension of WRED. Instead of dropping the packets when the
average queue length crosses the minimum threshold values, ECN marks the Congestion Experienced (CE) bit of the ECN
field in a packet as ECN-capable traffic (ECT).
1. Configure a WRED profile in CONFIGURATION mode.
OS10(config)# wred example-wred-prof
2. Configure WRED threshold parameters for different colors in WRED CONFIGURATION mode.
OS10(config-wred)# random-detect color yellow minimum-threshold 100 maximum-threshold
300 drop-probability 40
3. Configure the exponential weight value for the WRED profile in WRED CONFIGURATION mode.
OS10(config-wred)# random-detect weight 4
4. Enable ECN.
OS10(config-wred)# random-detect ecn
5. Enable WRED/ECN on a queue.
OS10(config)# class-map type queuing example-cmap-wred
OS10(config-cmap-queuing)# match queue 2
OS10(config-cmap-queuing)# exit
OS10(config)# policy-map type queuing example-pmap-wred
OS10(config-pmap-queuing)# class example-cmap-wred
OS10(config-pmap-c-que)# random-detect example-wred-prof
6. Enable WRED/ECN on a port.
OS10(config)# interface ethernet 1/1/1
OS10(conf-if-eth1/1/1)# random-detect example-wred-prof
7. Enable WRED/ECN on a service-pool.
OS10(config)# system qos
OS10(config-sys-qos)# random-detect pool 0 example-wred-prof
NOTE:
On the S4200–ON Series platform, enable ECN globally only. Also, apply ECN configurations only at the queue level.
You cannot configure ECN at the interface or service-pool levels. If you try to apply the ECN configuration at the interface
or service-pool levels, the configuration is not accepted.
To enable ECN globally:
1. Configure a WRED profile in CONFIGURATION mode.
OS10(config)# wred example-wred-prof-1
Quality of service
1553