Monday, September 8, 2014

Troubleshooting CRC and input errors with Nexus 5000 in transit path

When you encounter CRC (input) errors on an upstream switch interface, and you have a Nexus 5000 in the transit path (downstream), MTU Stomping by the latter is likely a factor.

On a Nexus 5000 switch, when a frame is received on a 10 Gb/s interface, it is considered to be in the cut-through path.  If there is no "network qos" type policy with jumbo frame support, and the hosts connected this switch generate jumbo frames, we will encounter the following situation:

There are logical and physical causes for the Nexus 5000 to drop a frame. There are also situations when a frame cannot be dropped because of the cut-through nature of the switch architecture. If a drop is necessary, but the frame is being switched in a cut-through path, then the only option is to stomp the Ethernet frame check sequence (FCS). Stomping a frame involves setting the FCS to a known value that does not pass a CRC check. This causes subsequent CRC checks to fail later in the path for this frame. A downstream store-and-forward device, or a host, will be able to drop this frame.

"show queueing interface ethernet x/y" will reveal the hardware MTU.  This setting is enforced by the application of a "network qos" type policy-map under "system qos" section of running configuration.

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