Administrator Guide

● If WRED/ECN is enabled on the global service-pool with threshold values and if it is not enabled on the queues, WRED/ECN
are not effective based on global service-pool WRED thresholds. The queue on which the traffic is scheduled must contain
WRED/ECN settings, which are enabled for WRED, to be valid for that traffic.
● 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 66. 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
DellEMC(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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