Configuring And Applying A Wred Table - HP 5920 Configuration Manual

sender proactively slow down the packet sending rate or decrease the window size of packets. This better
utilizes the network resources.
RFC 2482 defined an end-to-end congestion notification mechanism named Explicit Congestion
Notification (ECN). ECN uses the DS field in the IP header to mark the congestion status along the packet
transmission path. A ECN-capable terminal can determine whether congestion occurs on the
transmission path according to the packet contents, and then adjusts the packet sending speed to avoid
deteriorating congestion. ECN defines the last 2 bits (ECN field) in the DS field of the IP header as
follows:
Bit 6 indicates whether the sending terminal device supports ECN, and is called the "ECN-Capable
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Transport (ECT)" bit.
• Bit 7 indicates whether the packet has experienced congestion along the transmission path, and is
called the "Congestion Experienced (CE)" bit.
For more information about the DS field, see "Appendix."
In actual applications, the packets with ECT set to 1 and CE set to 0 and the packets with ECT set to 0
and CE set to 1 are considered as packets that an ECN-capable endpoint transmits.
After you enable ECN on a device, congestion management processes packets as follows:
• When the average queue size is below the lower threshold, no packet is dropped, and the ECN
fields of packets are not identified or marked.
When the average queue size exceeds the lower threshold and is below the upper threshold, before
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the device drops a packet which should be dropped according to the drop probability, the device
examines the ECN field of the packet.
If the ECN field shows that the packet is sent out of ECN-capable terminal, the device sets both
the ECT bit and the CE bit to 1 and forwards the packet.
If the ECN field shows that the packet has experienced congestion along the transmission path
(both the ECT bit and the CE bit are 1), the device forwards the packet without modifying the
ECN field.
If both the ECT bit and the CE bit is 0, the device drops the packet.
• When the average queue size exceeds the upper threshold, the device drops the packet, no matter
whether the packet is sent out of an ECN-capable terminal.
ECN is enabled on a per-queue basis. That is, you can configure the switch to identify and mark the ECN
fields of packets for a specific queue.

Configuring and applying a WRED table

The switch supports the queue-based WRED table. That is, you can configure separate drop parameters
for different queues. When congestion occurs, packets of a queue are randomly dropped based on drop
parameters of the queue.
Determine the following parameters before configuring WRED:
Upper threshold and lower threshold—When the average queue size is smaller than the lower
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threshold, no packet is dropped. When the average queue size is between the lower threshold and
the upper threshold, the packets are dropped based on the user-configured drop probability. When
the average queue size exceeds the upper threshold, subsequent packets are dropped.
Drop priority—A parameter used for packet drop. The value 0 corresponds to green packets, the
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value 1 corresponds to yellow packets, and the value 2 corresponds to red packets.
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