Connectivity Guide

Next-boot Settings : Enabled
Congestion avoidance
Congestion avoidance anticipates and takes necessary actions to avoid congestion. The following mechanisms avoid congestion:
• Tail drop—Packets are buered at trac queues. When the buers are exhausted or reach the congured threshold, excess packets
drop. By default, OS10 uses tail drop for congestion avoidance.
• Random early detection (RED)—In tail drop, dierent ows are not considered in buer utilization. When multiple hosts start
retransmission, tail drop causes TCP global re-synchronization. Instead of waiting for the queue to get lled up completely, RED starts
dropping excess packets with a certain drop-probability when the average queue length exceeds the congured 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 dierent drop-probabilities and thresholds for each color — red, yellow, green
— of trac. You can congure the drop characteristics for three dierent ows by assigning the colors to the ow. Assign colors to a
particular ow or trac using various methods, such as ingress policing, qos input policy-maps, and so on.
• Explicit congestion notication (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 eld in a packet as ECN-
capable trac (ECT).
1 Congure a WRED prole in CONFIGURATION mode.
OS10(config)# wred wred_prof_1
2 Congure WRED threshold parameters for dierent colors in WRED CONFIGURATION mode.
OS10(config-wred)# random-detect color yellow minimum-threshold 100 maximum-threshold 300
drop-probability 40
3 Congure the exponential weight value for the WRED prole 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 c1
OS10(config-cmap-queuing)# match queue 2
OS10(config-cmap-queuing)# exit
OS10(config)# policy-map type queuing p1
OS10(config-pmap-queuing)# class c1
OS10(config-pmap-c-que)# random-detect wred_prof_1
6 Enable WRED/ECN on a port.
OS10(config)# interface ethernet 1/1/1
OS10(conf-if-eth1/1/1)# random-detect wred_prof_1
7 Enable WRED/ECN on a service-pool.
OS10(config)# system qos
OS10(config-sys-qos)# random-detect pool 0 wred_prof_1
NOTE
: On the S4200–ON Series platform, enable ECN globally only. Also, apply ECN congurations only at the queue level. You
cannot congure ECN at the interface or service-pool levels. If you try to apply the ECN conguration at the interface or
service-pool levels, the conguration is not accepted.
1 Congure a WRED prole in CONFIGURATION mode.
OS10(config)# wred wred_prof_1
2 Congure WRED threshold parameters for dierent colors in WRED CONFIGURATION mode.
OS10(config-wred)# random-detect color yellow minimum-threshold 100 maximum-threshold 300
drop-probability 40
3 Congure the exponential weight value for the WRED prole in WRED CONFIGURATION mode.
OS10(config-wred)# random-detect weight 4
4 Congure the ECN threshold parameters in WRED CONFIGURATION mode.
OS10(config-wred)#random-detect ecn minimum-threshold 100 maximum-threshold 300 drop-
probability 40
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Quality of service