Configuration Guide User guide

50 FastIron Configuration Guide
53-1002494-02
Basic port parameter configuration
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 13.
If necessary, you can change the total buffer limits and the XON and XOFF default thresholds. Refer
to “Changing the total buffer limits” on page 52 and “Changing the XON and XOFF thresholds” on
page 51, respectively.
Configuration notes and feature limitations for
symmetric flow control
Note the following configuration notes and feature limitations before enabling symmetric flow
control.
• Symmetric flow control is supported on FCX and ICX devices only. It is not supported on other
FastIron models.
• Symmetric flow control is supported on all 1G and 10G data ports on FCX and ICX devices.
• 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 FCX.
By default, 802.3x flow control is enabled, but can be disabled with the no flow-control
command.
TABLE 13 XON and XOFF default thresholds
Limit when Jumbo disabled /
% of buffer limit
Limit when Jumbo enabled /
% of buffer limit
1G ports
Total buf fers 272 272
XOFF 240 / 91% 216 / 82%
XON 200 / 75% 184 / 70%
10G ports
Total buffers 416 416
XOFF 376 / 91% 336 / 82%
XON 312 / 75% 288 / 70%