Brocade Communications Systems Brocade ICX 6650 - Manual (Page 56)

38 Brocade ICX 6650 Administration Guide
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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.
NOTE
In Brocade ICX 6650, you cannot change the default XON and XOFF values.
Each 1G , 10G, and 40G port is configured with a default total number of buffers as well as a
default XOFF and XON threshold.
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 Brocade ICX 6650 devices.
• Symmetric flow control is supported on all 1 Gbps, 10 Gbps, and 40 Gbps data ports.
• To use this feature, 802.3x flow control must be enabled globally and per interface on the
Brocade ICX 6650 device. 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 command qd-descriptor and qd-buffer)
- Buffer profiles (CLI command buffer-profile port-region) is not supported for scheduler
profiles.
- DSCP-based QoS (CLI command trust dscp)
• 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.
TABLE 9 XON and XOFF default thresholds ( Apply to both Jumbo or non-Jumbo mode)
Limit when Jumbo disabled /
% of buffer limit Limit when Jumbo enabled /
% of buffer limit
1G or 10G ports
Total buffers 256 256
XOFF 192 (78%) 192 (78%)
XON 136 (56%) 136 (56%)
40G ports
Total buffers 960 960
XOFF 832 (87%) 832 (87%)
XON 720 (75%) 720 (75%)