Dell Brocade DCX, Configuration Manual

Page: 549 / 1436

BigIron RX Series Configuration Guide 477
53-1001810-01
Configuring packet drop priority using WRED
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Lower Wq values cause the avg-q-size to lean towards the statistical average queue size, reducing
WRED's sensitivity to the current state of the queue and thus reduce WRED's effectiveness. On the
other hand, higher Wq values cause the avg-q-size to lean towards the instantaneous queue size,
which exposes WRED to any change in the instantaneous queue size and thus may cause WRED to
overreact in cases of bursts. Thus, the value of Wq should be carefully chosen according to the
application at hand.
Calculating packets that are dropped
The Pdrop value, as calculated in the following equation, is the probability that a packet will be
dropped in a congested switch.
Using WRED with rate limiting
When rate limiting is configured on a device, it directs the switch to drop traffic indiscriminately
when the configured average-rate and maximum-burst thresholds are exceeded. If rate limiting is
configured with WRED, the traffic that exceeds these thresholds can be subjected to the WRED
algorithm which drops packets selectively by priority.
In this configuration, packets that exceed the thresholds established by the rate limiting
configuration are marked as either exceeding the average-rate or maximum-burst threshold. This
marking is then used to select a WRED configuration that determines which packets to drop.
Configuring packet drop priority using WRED
For a description of WRED, refer to “Determining packet drop priority using WRED” on page 475.
This section describes how to configure the parameters described in that section to enable the use
of WRED on a
device
.
To configure WRED, you must configure the following parameters:
•
“Enabling WRED”
•
“Setting the averaging-weight (Wq) parameter”
•
“Configuring the drop precedence parameters”
Enabling WRED
WRED must be enabled on any forwarding queue that you want it to operate on. To enable WRED
for the forwarding queue 3, enter the following command.
BigIron RX(config)#qos queue-type 3 wred enable
Syntax: [no] qos queue-type <queue-number> wred enable
pkt-size (avg-q-size - min-avg-q size)
Pdrop = ----------------- * Pmax * -----------------------------------------
pkt-size-max (max-avg-q-size - min-avg-q size)