Foundry Networks Switch and Router Installation And Configuration Manual page 340

Foundry Switch and Router Installation and Configuration Guide
NOTE: The weighted fair queuing method is based on packet-level scheduling. As a result, a queue's bandwidth
percentage does not necessarily reflect the exact bandwidth share the queue receives. This is due to the effects
of variable size packets.
USING THE CLI
To change the minimum guaranteed bandwidth percentages of the queues, enter commands such as the
following. Note that this example uses the default queue names.
BigIron(config)# qos profile qosp3 75 qosp2 10 qosp1 10 qosp0 5
Profile qosp3
Profile qosp2
Profile qosp1
Profile qosp0
BigIron(config)# write memory
Notice that the CLI displays the percentages you request and the percentages the device can provide based on
your request. The values are not always the same, as explained below.
Syntax: [no] qos profile <queue> <percentage> <queue> <percentage> <queue> <percentage>
<queue> <percentage>
Each <queue> parameter specifies the name of a queue. You can specify the queues in any order on the
command line, but you must specify each queue.
The <percentage> parameter specifies a number for the percentage of the device's outbound bandwidth that are
allocating to the queue.
NOTE: The percentages you enter must equal 100. Also, the percentage for the premium queue (the highest
priority queue) must be at least 50.
If you enter percentages that are less than the minimum percentages supported for a queue, the CLI recalculates
the percentages to fall within the supported minimums. Here is an example. In this example, the values entered
for all but the best-effort queue (the lowest priority queue) are much lower than the minimum values supported for
those queues.
BigIron(config)# qos qosp3 1 qosp2 1 qosp1 2 qosp0 96
Warning - qosp3 bandwidth should be at least 50%
bandwidth scheduling mechanism: weighted priority
Profile qosp3
Profile qosp2
Profile qosp1
Profile qosp0
This example shows the warning message that is displayed if you enter a value that is less than 50% for the
premium queue. This example also shows the recalculations performed by the CLI. The CLI must normalize the
values because the weighted fair queuing algorithm and queue hardware require specific minimum bandwidth
allocations. You cannot configure the hardware to exceed the weighted fair queuing limitations.
The CLI normalizes the percentages you enter by increasing the percentages as needed for queues that have
less than the minimum percentage, converting the percentages to weights (which the weighted fair queuing
algorithm uses), and applying the following equations to calculate the percentages:
qosp3 = w3 / (w3 + 1)
qosp2 = (1 – qosp3) * w2 / (w2 + 1)
qosp1 = (1 – qosp3 – qosp2) * w1 / (w1 + 1)
qosp0 = 1 – qosp3 – qosp2 – qosp1
The value "w" stands for "weight". Thus, these calculations determine the weights that the weighted fair queuing
algorithm will use for each queue.
11 - 8
: PREMIUM bandwidth requested
: HIGH bandwidth requested
: NORMAL bandwidth requested
: BEST-EFFORT bandwidth requested
: PREMIUM bandwidth requested
: HIGH bandwidth requested
: NORMAL bandwidth requested
: BEST-EFFORT bandwidth requested
75% calculated 75%
10% calculated 13%
10% calculated 8%
5% calculated 4%
1% calculated 50%
1% calculated 25%
2% calculated 13%
96% calculated 12%
December 2000