Brocade Converged Enhanced Ethernet Administrator's Guide v6.1.2_cee (53-1001258-01, June 2009)

112 Converged Enhanced Ethernet Administrator’s Guide
53-1001258-01
Congestion control
8
Ethernet pause
Ethernet Pause is an IEEE 802.3 standard mechanism for back pressuring a neighboring device.
Pause messages are sent by utilizing the optional MAC control sublayer. A Pause frame contains a
2-byte pause number, which states the length of the pause in units of 512 bit times. When a device
receives a Pause frame, it must stop sending any data on the interface for the specified length of
time, once it completes transmission of any frame in progress. You can use this feature to reduce
Ethernet packet losses by using a standardized mechanism. However the Pause mechanism does
not have the ability to selectively back pressure data sources multiple hops away, or exert any
control per VLAN or per priority so it is disruptive to all traffic on the link.
Ethernet Pause includes the following features:
• All configuration parameters can be specified independently per interface.
• Pause On/Off can be specified independently for TX and RX directions. No support is provided
for auto-negotiation.
• Pause generation is based on input (receive) queueing. Queue levels are tracked per input
port. You can change the high-water and low-water threshold for each input port. When the
instantaneous queue depth crosses the high-water mark then a Pause is generated. If any
additional packets are received and the queue length is still above the low-water mark then
additional Pause’s are generated. Once the queue length drops below the low-water mark then
Pause generation ceases.
• A Pause that is received and processed halts transmission of the output queues associated
with the port for the duration specified in the Pause frame.
Enabling an Ethernet Pause
Example of enabling an interface 802.3x Pause flow control TX and RX.
switch:admin>cmsh
switch>enable
switch#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
switch(config)#interface tengigabitethernet 0/2
switch(conf-if-te-0/2)#qos flowcontrol tx on rx on
switch(conf-if-te-0/2)#exit
switch(config)#exit
switch#
Step Task Command
1 Enter global configuration mode.
switch#configure terminal
2 Specify the 10-gigabit Ethernet interface. In
this example, 0/2 is used.
switch(config)#interface
tengigabitethernet 0/2
3 Enable an Ethernet Pause on the interface for
both TX and RX traffic.
switch(conf-if-te-0/2)#qos
flowcontrol tx on rx on