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Dell Converged Enhanced Ethernet Administrator’s Guide 101
53-1002116-01
Congestion control
10
Changing the Tail Drop threshold
To change the Tail Drop threshold, perform the following steps from Privileged EXEC mode.
1. Enter global configuration mode.
switch#configure terminal
2. Change the Tail Drop threshold for each multicast traffic class. In this example, 1000pkt is
used.
switch(config)#qos rcv-queue multicast threshold 1000 1000 1000 1000 1000 1000
1000 1000
The following example increases multicast frame expansion Tail Drop Threshold to 1000pkt for
each multicast Traffic Class.
switch:admin>cmsh
switch>enable
switch#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
switch(config)#qos rcv-queue multicast threshold 1000 1000 1000 1000 1000 1000
1000 1000
switch(config)#end
3. Enter the copy command to save the running-config file to the startup-config file.
switch#copy running-config startup-config
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 frame 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 frames are received and the queue length is still above the low-water mark then
additional Pauses 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.