Users Guide

• When WRED is configured on the global service-pool (regardless of whether ECN on global service-pool is configured), and one or
more queues have WRED enabled and ECN disabled, WRED is effective for the minimum of the thresholds between the queue
threshold and the service-pool threshold.
• When WRED is configured on the global service-pool (regardless of whether ECN on global service-pool is configured), and one or
more queues are enabled with both WRED and ECN, ECN marking takes effect. The packets are ECN marked up to shared- buffer
limits as determined by the shared-ratio for that global service-pool.
WRED/ECN configurations for the queues that belong to backplane ports are common to all the backplane ports and cannot be specified
separately for each backplane port granularity. This behavior occurs to prevent system-level complexities in enabling this support for
backplane ports. Also, WRED/ECN is not supported for multicast packets.
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 77. Scenarios of WRED and ECN Configuration
Queue Configuration Service-Pool
Configuration
WRED Threshold
Relationship
Q threshold = Q-T,
Service pool threshold =
SP-T
Expected Functionality
WRED ECN WRED ECN
0 0 X X X WRED/ECN not applicable
1 0 0 X X Queue based WRED,
No ECN marking
1 X Q-T < SP-T
SP-T < Q-T SP based WRED,
No ECN marking
1 1 0 X X Queue-based ECN marking above queue threshold.
ECN marking to shared buffer limits of the service-pool and
then packets are tail dropped.
1 X Q-T < SP-T
SP-T < Q-T Same as above but ECN marking starts above SP-T.
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 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
Quality of Service (QoS)
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