Technical data

FastIron Ethernet Switch Administration Guide 51
53-1002637-02
Basic port parameter configuration
2
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 10.
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 53 and “Changing the XON and XOFF thresholds” on
page 52, 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 FCX and
ICX devices. By default, 802.3x flow control is enabled, but can be disabled with the no
flow-control command.
• The following QoS features are not supported together with symmetric flow control:
- Dynamic buffer allocation (CLI commands qd-descriptor and qd-buffer)
- Buffer profiles (CLI command buffer-profile port-region)
- DSCP-based QoS (CLI command trust dscp)
TABLE 10 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%