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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 two 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 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 "Appendixes."
In actual ap
plications, 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 limit, no packet is dropped, and the ECN fields
of packets are not identified or marked.
• When the average queue size exceeds the lower limit and is below the upper limit, before 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 are 0s, the device drops the packet.
• When the average queue size exceeds the upper limit, the device drops the packet, regardless
of whether the packet is sent out from an ECN-capable terminal.
ECN is enabled on a per-queue basis. You can configure the switch to identify and mark the ECN
fields of packets for a specific queue.
Configuring and applying a WRED table
You can apply WRED tables to both Layer 2 and Layer 3 Ethernet interfaces. The term "interface" in
this section collectively refers to these two types of interfaces. You can use the port link-mode
command to configure an Ethernet port as a Layer 2 or Layer 3 interface (see Layer 2—LAN
Switching Configuration Guide).
The switch supports the queue-based WRED table. 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 limit and lower limit—When the average queue size is smaller than the lower limit,
packets are not dropped. When the average queue size is between the lower limit and the
upper limit, the packets are dropped based on the user-configured drop probability. When the
average queue size exceeds the upper limit, subsequent packets are dropped.
• Drop precedence—A parameter used for packet drop. The value 0 corresponds to green
packets, the value 1 corresponds to yellow packets, and the value 2 corresponds to red packets.
Red packets are dropped preferentially.
• Exponent for average queue size calculation—The greater the exponent, the less sensitive
the average queue size is to real-time queue size changes. The formula for calculating the
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