Dell PowerConnect B-FCXs, Configuration Manual

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42 PowerConnect B-Series FCX Configuration Guide
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Configuring basic port parameters
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The following QoS features are not supported together with symmetric flow control:
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Dynamic buffer allocation (CLI commands qd-descriptor and qd-buffer)
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Buffer profiles (CLI command buffer-profile port-region)
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DSCP-based QoS (CLI command trust dscp)
NOTE
Although the above QoS features are not supported with symmetric flow control, the CLI will
still accept these commands. The last command issued will be the one placed into effect on
the device. For example, if trust dscp is enabled after symmetric-flow-control is enabled,
symmetric flow control will be disabled and trust dscp will be placed into effect. Make sure you
do not enable incompatible QoS features when symmetric flow control is enabled on the
device.
•
Head of Line (HOL) blocking may occur when symmetric flow control is enabled. This means
that a peer can stop transmitting traffic streams unrelated to the congestion stream.
Enabling and disabling symmetric flow control
By default, symmetric flow control is disabled and tail drop mode is enabled. However, because
flow control is enabled by default on all full-duplex ports, these ports will always honor received
802.3x Pause frames, whether or not symmetric flow control is enabled.
To enable symmetric flow control globally on all full-duplex data ports of a standalone unit, enter
the following command.
PowerConnect(config)# symmetric-flow-control enable
To enable symmetric flow control globally on all full-duplex data ports of a particular unit in an
IronStack, enter a command such as the following.
PowerConnect(config)# symmetric-flow-control enable unit 4
Syntax: [no] symmetric-flow-control enable [unit < stack-unit >]
The <stack-unit> parameter specifies one of the units in a stacking system.
Master/Standby/Members are examples of a stack-unit
To disable symmetric flow control once it has been enabled, use the no form of the command.
Changing the XON and XOFF thresholds
This section describes how to change the XON and XOFF thresholds described in “About XON and
XOFF thresholds” on page 41.
To change the thresholds for all 1G ports, enter a command such as the following.
PowerConnect(config)# symmetric-flow-control set 1 xoff 91 xon 75
To change the thresholds for all 10G ports, enter a command such as the following.
PowerConnect(config)# symmetric-flow-control set 2 xoff 91 xon 75
In the above configuration examples, when the XOFF limit of 91% is reached or exceeded, the Dell
PowerConnect device will send PAUSE frames to the sender telling it to stop transmitting data
temporarily. When the XON limit of 75% is reached, the Dell PowerConnect device will send PAUSE
frames to the sender telling it to resume sending data.