Configuring Wred And Ecn Attributes - Dell S4048–ON Configuration Manual

The following table describes the WRED and ECN operations that occur for various scenarios of WRED and ECN configuration on
the queue and service pool. (X denotes not-applicable in the table, 1 indicates that the setting is enabled, 0 represents a disabled
setting. )
Table 76. Scenarios of WRED and ECN Configuration
Queue Service-Pool
Configuration Configuration
WRED ECN WRED
0 0 X
1 0 0
1
1 1 0
1

Configuring WRED and ECN Attributes

The functionality to configure a weight factor for the WRED and ECN functionality for backplane ports is supported on the Z9000
platform.
WRED drops packets when the average queue length exceeds the configured threshold value to signify congestion. Explicit
Congestion Notification (ECN) is a capability that enhances WRED by marking the packets instead of causing WRED to drop them
when the threshold value is exceeded. If you configure ECN for WRED, devices employ this functionality of ECN to mark the
packets and reduce the rate of sending packets in a congested, heavily-loaded network.
To configure the weight factor for WRED and ECN capabilities, global buffer pools for multiple queues, and associating a service
class with ECN marking, perform the following:
1. Configure the weight factor for the computation of average-queue size. This weight value applies to front-end ports.
QOS-POLICY-OUT mode
Dell(conf-qos-policy-out)#wred—profile weight number
2. Configure a WRED profile, and specify the threshold and maximum drop rate.
WRED mode
Dell(conf-wred) #wred—profile thresh-1
Dell(conf-wred) #threshold min 100 max 200 max-drop-rate 40
3. Configure another WRED profile, and specify the threshold and maximum drop rate.
WRED mode
Dell(conf-wred) #wred—profile thresh-2
698
Quality of Service (QoS)
WRED Threshold
Relationship
Q threshold = Q-T,
Service pool threshold
= SP-T
ECN
X X
X X
X Q-T < SP-T
SP-T < Q-T
X X
X Q-T < SP-T
SP-T < Q-T
Expected Functionality
WRED/ECN not applicable
Queue based WRED,
No ECN marking
SP based WRED,
No ECN marking
Queue-based ECN marking above queue threshold.
ECN marking to shared buffer limits of the service-pool and
then packets are tail dropped.
Same as above but ECN marking starts above SP-T.