Guidelines And Limitations For Congestion Avoidance - Cisco MDS 9000 Series Configuration Manual

Guidelines and Limitations for Congestion Avoidance

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ERROR STATISTICS INFORMATION FOR DEVICE DEVICE: FCMAC
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fc1/8
fc1/8
fc1/5
fc1/3
fc1/8
Guidelines and Limitations for Congestion Avoidance
The default value for system timeout congestion-drop is 500 ms.
System timeout no-credit-drop is disabled by default. When configured, this feature reduces the effects of
slow drain in the fabric. However, if it is configured to a value that is too low, it can cause disruption. The
disruption is caused because many frames are discarded when a device withholds credits for even a short
duration. The lower the value, the quicker it can start discarding frames that are queued from an upstream ISL
to this (slow) port. This will relieve the back pressure or congestion on that ISL and allow other normally
performing devices to continue their operation. The actual value chosen is fabric and implementation dependent.
Following are some guidelines for choosing the system timeout congestion-drop value:
• 200 ms—Safe value for most fabric
• 100 ms—Aggressive value
• 50 ms—Very aggressive value
Generally, before configuring the no-credit-drop value, the switches should be checked for the presence of
large amounts of continuous time at zero Tx credits. The show logging onboard start time
mm/dd/yy-hh:mm:ss error-stats command can be run looking for instances of the
FCP_SW_CNTR_TX_WT_AVG_B2B_ZERO counter indicating 100ms intervals at zero credits. Additionally,
the port-monitor tx-credit-not-available and the show system internal snmp credit-not-available commands
will show similar information. Only when the fabric only shows very limited amounts of 100ms at zero Tx
credits should no-credit-drop be considered. If there are large amounts of ports with 100ms at zero Tx credits,
then the problems with those end devices should be investigated and resolved prior to configuring
no-credit-drop.
Note No-credit-drop can only be configured on ports that are classified logical-type edge. These are typically F
ports.
Slowport-monitor, if configured, must have a value lower than no-credit-drop since it will only indicate a
slow port if the port has no credits for at least the amount of time configured and there are frames queued for
transmit. Since no-credit-drop will drops any frames queued for transmit, if no-credit-drop is configured for
Cisco MDS 9000 Series Interfaces Configuration Guide, Release 8.x
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Interface |
Range |
|
|F16_TMM_TOLB_TIMEOUT_DROP_CNT
|F16_TMM_TOLB_TIMEOUT_DROP_CNT
|F16_TMM_TOLB_TIMEOUT_DROP_CNT
|F16_TMM_TOLB_TIMEOUT_DROP_CNT
|F16_TMM_TOLB_TIMEOUT_DROP_CNT
Congestion Detection, Avoidance, and Isolation
Error Stat Counter Name
| | Time Stamp
| Count |MM/DD/YY HH:MM:SS
| |
|743 |01/12/18 11:37:55
|626 |12/20/17 06:31:47
|627 |12/20/17 06:31:47
|556 |12/20/17 06:31:47
|623 |12/20/17 04:05:05