Configuring Congestion Avoidance; Configuring The Congestion Drop Timeout Value For Fcoe - Cisco MDS 9000 Series Configuration Manual

Congestion Detection, Avoidance, and Isolation

Configuring Congestion Avoidance

The following features can be configured for congestion avoidance:
• Congestion-drop
• No-credit-drop
• Pause-drop
• Port-monitor portguard action for congestion avoidance (errdisable and flap)

Configuring the Congestion Drop Timeout Value for FCoE

When an FCoE frame takes longer than the congestion drop timeout period to be transmitted by the egress
port, the frame is dropped. This dropping of frames is useful in controlling the effect of slow egress ports that
are paused almost continuously (long enough to cause congestion), but not long enough to trigger the pause
timeout drop. Frames dropped due to the congestion drop threshold are counted as egress discards against the
egress port. Egress discards release buffers in the upstream ingress ports of a switch, allowing the unrelated
flows to move continuously through the ports.
The congestion drop timeout value is 500 ms by default for all port types. We recommend that you retain the
default timeout for core ports, and consider configuring a lower value for edge ports. The congestion drop
timeout value should be equal to or greater than the pause drop timeout value for that port type.
To configure the congestion drop timeout value for FCoE, perform these steps:
Step 1
Enter configuration mode:
switch# configure terminal
Step 2 Depending on the Cisco MDS NX-OS release version you are using, use one of the following commands to configure
the system-wide FCoE congestion drop timeout, in milliseconds, for core or edge ports:
• Cisco MDS NX-OS Release 8.1(1) and earlier releases
switch(config)# system default interface congestion timeout milliseconds mode {core | edge}
The FCoE congestion drop timeout range is from 100 to 1000 ms.
Note To prevent premature packet drops, the minimum value recommended for FCoE congestion drop timeout
is 200 ms.
• Cisco MDS NX-OS Release 8.2(1) and later releases
switch(config)# system timeout fcoe congestion-drop {milliseconds | default} mode {core | edge}
The FCoE congestion drop timeout range is from 200 to 500 ms.
In Cisco MDS NX-OS Release 8.1(1) and earlier releases, the FCoE congestion drop timeout value could
Note
be configured to as low as 100 ms. However, under certain circumstances configuring a congestion drop
timeout value of 100 ms led to premature packet drops. In Cisco MDS NX-OS 8.2(1) and later releases,
the minimum congestion drop timeout value was set to 200 ms to prevent premature packet drops. Therefore,
we do not recommended that you specify a congestion drop timeout value of less than 200 ms in Cisco
MDS NX-OS Release 8.1(1) and earlier releases.
Cisco MDS 9000 Series Interfaces Configuration Guide, Release 8.x
Configuring Congestion Avoidance
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