Information About Congestion Avoidance - Cisco MDS 9000 Series Configuration Manual

Congestion Detection, Avoidance, and Isolation

Information About Congestion Avoidance

Congestion avoidance focuses on minimizing or completely avoiding the congestion that results from frames
being queued to congested ports.
Cisco MDS switches have multiple features designed to void congestion in SAN:
• Congestion-drop timeout threshold (Fibre Channel and FCoE): The congestion-drop timeout threshold
• No-credit-drop timeout threshold (Fibre Channel only): No-credit-drop timeout threshold is used to time
• Pause-drop timeout threshold (FCoE only): Pause-drop timeout threshold is used to time when a FCoE
Feature Name
Port monitor's tx-datarate counter
Port monitor's tx-discards counter
Port monitor's tx-slowport-count
counter
Port monitor's
tx-slowport-oper-delay counter
Port monitor's txwait counter
determines the amount of time a queued Fibre Channel or FCoE frame will stay in the switch awaiting
transmission. Once the threshold is reached the frame is discarded as a timeout drop. The lower the value
the quicker these queued frames are dropped and the result buffer freed. This can relieve some back
pressure in the switch, especially on ISLs. By default it is 500 ms but can be configured as low as 200
ms in 1 ms increments. It is configured using the system timeout congestion-drop (Fibre Channel) and
system timeout fcoe congestion-drop (FCoE) commands.
when a Fibre Channel port is at zero Tx credits. Once a Fibre Channel port hits zero Tx credits the timer
is started. If the configured threshold is reached then all frames queued to that port will be dropped
regardless of their actual age in the switch. Furthermore, as long as the port remains at zero Tx credits,
all newly arriving frames are immediately dropped. This can have a dramatic effect on reliving congestion
especially on upstream ISLs. This allows unrelated flows to move continuously. This is off by default.
If configured, it should be set to a value that is lower than the configured (or defaulted) Fibre Channel
congestion-drop timeout. It is configured via the system timeout no-credit-drop command. The no-credit
timeout functionality is only used for edge ports because these ports are directly connected to the
slow-drain devices.
port is in a continuous state of Rx pause (unable to transmit). After an FCoE port receives a PFC pause
with a non-zero quanta, the timer is started. If the port continues to receive PFC pauses with a non-zero
quanta such that it remains in the Rx pause state continuously for the pause-drop threshold, then all
frames queued to that port will be dropped regardless of their actual age in the switch. Furthermore, as
long as the port remains in a Rx pause state, all newly arriving frames are immediately dropped. This
Description
Tx-datarate counter represents the transmit frame rate in bytes per
seconds.
Tx-discards counter represents the total number of frames that are
dropped at egress due to timeout, abort, offline, and so on.
Tx-slowport-count counter represents the number of times slow
port events were detected by a port for the configured
slowport-monitor timeout. This counter is applicable only for
Generation 3 modules.
Tx-slowport-oper-delay counter captures average credit delay (or
R_RDY delay) experienced by a port. The value is in milliseconds.
TxWait counter is an aggregate time-counter that counts transmit
wait time of a port. Transmit wait is a condition when a port
experiences no transmit credit available (Tx B2B = 0) and frames
are waiting for transmission.
Cisco MDS 9000 Series Interfaces Configuration Guide, Release 8.x
Information About Congestion Avoidance
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