Brocade Network OS Administrator’s Guide v4.1.1 - Manual (Page 475)

device receives a PAUSE frame, it must stop sending any data on the interface for the specified length
of time, once it completes the 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 to exert any control
per VLAN or per priority, so it is disruptive to all traffic on the link.
Ethernet Pause features
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
disabling autonegotiation.
• Pause generation is based on input (receive) queueing. Queue levels are tracked per input port.
When the instantaneous queue depth crosses the high-water mark, then a PAUSE frame is
generated. If any additional frames are received and the queue length is still above the low-water
mark, then additional PAUSE frames are generated. Once the queue length drops below the low-
water mark, then the generation of PAUSE frames ceases.
• A PAUSE frame that is received and processed halts transmission of the output queues associated
with the port for the duration specified in the PAUSE frame.
1-Gbps pause negotiation
When a 1-Gbps local port is already online, and the qos flowcontrol command is issued, the pause
settings take effect immediately on that local port. However, when the link is toggled, pause is
renegotiated. The local port will advertise the most recent qos flowcontrol settings. After
autonegotiation completes, the local port pause settings may change, depending on the outcome of the
pause negotiation, per 802.3 Clause 28B, as shown in the table below.
Pause negotiation results TABLE 76
Advertised LOCAL cfg Advertised REMOTE cfg Negotiated result
Rx=off Tx=on Rx=on Tx=on asymmetrical: LOCAL Tx=on –> pause –> REMOTE Rx=on
Rx=on Tx=on Rx=off Tx=on asymmetrical: LOCAL Rx=on <– pause <– REMOTE Tx=on
Rx=on Tx=n/a Rx=on Tx=n/a symmetrical: LOCAL Tx/Rx=on <– pause –> REMOTE Tx/
Rx=on
Rx=n/a Tx=n/a Rx=off Tx=off disable pause both sides
Ethernet Priority Flow Control
Ethernet Priority Flow Control (PFC) is a basic extension of 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 number is
applied to the corresponding Ethernet priority/class level. For example, the Pause number 0 applies to
priority zero, Pause number 1 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 Pause features
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