Dell PowerConnect B-FCXs Configuration Manual page 83

Symmetric flow control addresses the requirements of a lossless service class in an Internet Small
Computer System Interface (iSCSI) environment. It is supported on FCX standalone units as well as
on all FCX units in an IronStack.
About XON and XOFF thresholds
An 802.3x PAUSE frame is generated when the buffer limit at the ingress port reaches or exceeds
the port's upper watermark threshold (XOFF limit). The PAUSE frame requests that the sender stop
transmitting traffic for a period of time. The time allotted enables the egress and ingress queues to
be cleared. When the ingress queue falls below the port's lower watermark threshold (XON limit),
an 802.3x PAUSE frame with a quanta of 0 (zero) is generated. The PAUSE frame requests that the
sender resume sending traffic normally.
Each 1G and 10G port is configured with a default total number of buffers as well as a default XOFF
and XON threshold. The defaults are different for 1G ports versus 10G ports. Also, the default XOFF
and XON thresholds are different for jumbo mode versus non-jumbo mode. The defaults are shown
in Table 9.
TABLE 9
1G ports
Total buffers
XOFF
XON
10G ports
Total buffers
XOFF
XON
If necessary, you can change the total buffer limits and the XON and XOFF default thresholds. Refer
to "Changing the total buffer limits"
page 42, respectively.
Configuration notes and feature limitations for symmetric flow control
Note the following configuration notes and feature limitations before enabling symmetric flow
control.
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PowerConnect B-Series FCX Configuration Guide
53-1002266-01
XON and XOFF default thresholds
Symmetric flow control is supported on PowerConnect B-Series FCX devices only.
Symmetric flow control is supported on all PowerConnect B-Series FCX 1G and 10G data ports.
Symmetric flow control is not supported on stacking ports or across units in a stack.
To use this feature, 802.3x flow control must be enabled globally and per interface on the
PowerConnect B-Series FCX . By default, 802.3x flow control is enabled, but can be disabled
with the no flow-control command.
Configuring basic port parameters
Limit when Jumbo disabled /
% of buffer limit
272
240 / 91%
200 / 75%
416
376 / 91%
312 / 75%
on page 43 and "Changing the XON and XOFF thresholds"
Limit when Jumbo enabled /
% of buffer limit
272
216 / 82%
184 / 70%
416
336 / 82%
288 / 70%
2
on
41