Hewlett Packard Enterprise FlexFabric 5940 Series, Руководство по настройке

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In actual applications, the following packets are considered as packets that an ECN-capable
endpoint transmits:
•
Packets with ECT set to 1 and CE set to 0.
•
Packets with ECT set to 0 and CE set to 1.
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 is between the lower threshold and the upper threshold, the
device performs the following operations:
a.
Picks out packets to be dropped according to the drop probability.
b.
Examines the ECN fields of these packets and determines whether to drop these packets.
{
If the ECN field shows that the packet is sent out of ECN-capable terminal, the device
performs the following operations:
−
Sets both the ECT bit and the CE bit to 1.
−
Forwards the packet.
{
If both the ECT bit and the CE bit are 1 in the packet, the device forwards the packet without
modifying the ECN field. The combination of ECT bit 1 and CE bit 1 indicates that the
packet has experienced congestion along the transmission path.
{
If both the ECT bit and the CE bit is 0 in the packet, the device drops the packet.
•
When the average queue size exceeds the upper threshold, the device drops the packet,
regardless of whether the packet is sent from an ECN-capable terminal.
The switch supports enabling ECN on a per-queue basis.
Configuring and applying a queue-based WRED
table
The switch supports queue-based WRED tables. You can configure separate drop parameters for
different queues. When congestion occurs, packets of a queue are randomly dropped based on the
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 threshold, packets are not dropped. When the average queue size is between the lower
threshold and the upper threshold, the packets are dropped at random. The longer the queue,
the higher the drop probability. When the average queue size exceeds the upper threshold,
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
average queue size is:
Average queue size = ( previous average queue size x (1 – 2
–n
) ) + (current queue size x 2
–n
),
where n is the exponent.
•
Drop probability
—This parameter is used when a WRED table is configured. The drop
probability is expressed in percentage. The greater the percentage value, the greater the drop
probability.