Cisco MDS 9000 Series Configuration Manual page 154

Information About SAN Congestion Caused by Slow-Drain Devices
• PFC Pause (FCoE): A Buffer-to-Buffer credit. For more information, see
• Transitions to zero: A Virtual Link based Buffer-to-Buffer credit. From Cisco MDS NX-OS 8.1(1),
• PFC Pause (FCoE only): Priority Flow Control is a class-based flow control mechanism where
• Transitions to zero (Fibre Channel only): When the remaining Tx credit count hits zero, the Tx
• TxWait (Fibre Channel and FCoE): TxWait is a measure of time when a port cannot transmit when
• RxWait (FCoE only): RxWait is a measure of time where a port cannot receive frames. A port cannot
• TX Credit not Available (Fibre Channel only): Tx Credit not Available is a software counter that
• Timeout-drop (Fibre Channel and FCoE): A frame is dropped as a timeout drop when a received
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credit count is greater than zero. When the frame is transmitted, the remaining Tx credit count is
decremented by one. When the receiver of the frame processes the frame, it returns a credit that is
called Receiver Ready (R_RDY). When an R_RDY is returned, the frame sender increments the
remaining Tx credit count by one. If the remaining Tx credit count hits zero, no further frames can
be transmitted until an R_RDY is received.
credits (Fibre Channel only).
we introduced the Congestion-Isolation feature. This feature allows slow-drain devices to be isolated
to a slow traffic virtual link (VL2) on an ISL (E port). The ISL must be in the Extended Receiver
Ready (ER_RDY) mode for this feature to function. When an ISL is in ER_RDY mode, the link is
logically partitioned into four separate virtual links. ER_RDY contains the VL number indicating
which VL the BB credit is used for.
class-based pause frames are sent to stop the flow of data in one direction for a specific class of
service. PFC pause frames contain a value that is called a quanta. The quanta determines how long
a class of traffic is paused. The two types of PFC pause frames are nonzero quanta and zero quanta.
A PFC pause frame with a nonzero quanta signals the receiver to stop sending frames immediately
for a specified amount of time. A PFC pause frame with a zero quanta signals the receiver that it
can resume sending frames immediately. A PFC pause frame with a zero quanta can be called as
an unpause or resume.
transition to zero is incremented on the Tx side. On the Rx side (the side withholding the BB_credits),
the Rx transition to zero is incremented. It is important to understand that the amount of time actually
at zero remaining Tx credit count is not represented by this term. It could be for a short time that
does not affect performance or it could be for a longer time that affects performance. Because of
this condition, transitions to zero is not a good measure of congestion.
it has frames queued in it. A port cannot transmit if it is at zero remaining Tx credit count (Fibre
Channel) or if it has received a PFC pause frame. Each time TxWait increments, the port (or class)
is unable to transmit for 2.5 microseconds. TxWait value can be converted to seconds by multiplying
it by 2.5 and then dividing by 1,000,000.
receive frames if it has withheld all BB_credits from the transmitter (when a port is at zero remaining
Rx credit count) (Fibre Channel) or if it has transmitted a PFC pause frame (FCoE). Each time
RxWait increments, the port (or class) is unable to receive for 2.5 microseconds. RxWait can be
converted to seconds by multiplying it by 2.5 and then dividing by 1,000,000.
increments by one when the port is at zero remaining Tx credit count continuously for 100
milliseconds.
frame is unable to be transmitted out of the egress port in the configured congestion-drop threshold.
This condition is typically due to congestion at the egress port that is caused by a lack of Tx
BB_credits (Fibre Channel) or in an Rx Pause state (FCoE). The default timeout drop value is 500
milliseconds for both Fibre Channel and FCoE but can be configured to a value as low as 200
Congestion Detection, Avoidance, and Isolation
Buffer-to-Buffer (BB)