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Converged Enhanced Ethernet Administrator’s Guide 115
53-1002163-02
Congestion control
11
DRAFT: BROCADE CONFIDENTIAL
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.
Enabling Ethernet Pause
To enable Ethernet Pause, perform the following steps from privileged EXEC mode.
1. Enter global configuration mode.
switch#configure terminal
2. Specify the 10-gigabit Ethernet interface.
switch(config)#interface tengigabitethernet 0/2
3. Enable Ethernet Pause on the interface for both TX and RX traffic.
switch(conf-if-te-0/2)#qos flowcontrol tx on rx on
4. Enter the copy command to save the running-config file to the startup-config file.
switch(config)#do copy running-config startup-config
Ethernet Priority Flow Control
Ethernet Priority Flow Control (PFC) is a basic extension of the Ethernet Pause. The Pause MAC
control message is extended with eight 2-byte pause numbers and a bitmask to indicate which
values are valid. Each pause number is interpreted identically to the base Pause protocol; however
each is applied to the corresponding Ethernet priority / class level. For example, the Pause number
zero applies to priority zero, Pause number one applies to priority one, and so on. This addresses
one shortcoming of the Ethernet Pause mechanism, which is disruptive to all traffic on the link.
However, it still suffers from the other Ethernet Pause limitations.
Ethernet Priority Flow Control includes the following features:
• Everything operates exactly as in Ethernet Pause described above except there are eight
high-water and low-water thresholds for each input port. This means queue levels are tracked
per input port plus priority.
• Pause On/Off can be specified independently for TX and RX directions per priority.
• Pause time programmed into Ethernet MAC is a single value covering all priorities.