Users Guide

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
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
Dell(conf-wred) #threshold min 300 max 400 max-drop-rate 80
4. Create a global buffer pool that is a shared buffer pool accessed by multiple queues when the minimum guaranteed buffers for the
queue are consumed.
mode
Dell(conf) #service-pool wred green pool0 thresh-1 pool1 thresh-2
Dell(conf) #service-pool wred yellow pool0 thresh-3 pool1 thresh-4
Dell(conf) #service-pool wred weight pool0 11 pool1 4
Guidelines for Configuring ECN for Classifying and
Color-Marking Packets
Keep the following points in mind while configuring the marking and mapping of incoming packets using ECN fields in IPv4 headers:
• Currently Dell Networking OS supports matching only the following TCP flags:
○ ACK
○ FIN
○ SYN
○ PSH
○ RST
○ URG
In the existing software, ECE/CWR TCP flag qualifiers are not supported.
• Because this functionality forcibly marks all the packets matching the specific match criteria as ‘yellow’, Dell Networking OS does not
support Policer based coloring and this feature concurrently.
• If single rate two color policer is configured along with this feature, then by default all packets less than PIR would be considered as
“Green” But ‘Green’ packets matching the specific match criteria for which ‘color-marking’ is configured will be over-written and
marked as “Yellow”.
• If two rate three color policer is configured along with this feature then,
○ x < CIR – will be marked as “Green”
○ CIR < x< PIR – will be marked as “Yellow”
○ PIR < x – will be marked as “Red”
Quality of Service (QoS)
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