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I have a Cisco 3560 switch which is dropping some frames the frames that are dropped are seen in a SPAN session for the ingress port (monitor source) but not in a SPAN session for the egress It has a positive counter displayed with show controllers "Valid frames, too small" against the receive side of the ingress port.

I cannot yet find any Cisco references to define this value. The frames that are being dropped have a Wireshark-reported frame length of 214 octets.

Does anybody know of a reference where this value is defined? (or know what this value refers to)

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From this link I can see:

The number of frames received on an interface that are smaller than 64 bytes (or 68 bytes for VLAN-tagged frames) and that have valid FCS values. The frame size includes the FCS bits but excludes the frame header bits.

Now, why this should show up for 214-bytes frames is hard to say. Is it possible that the frames matching against this counter are not the same ones you see on wireshark?

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  • Thanks for that, I wonder is this the same as a runt, I always assumed a runt would have an invalid FCS. It is certainly possible that the frames matching this are not the ones I see in Wireshark. Either way I guess, the switch is duff. Either it's mis-reporting and dropping or it's dropping silently.
    – marctxk
    Dec 6, 2017 at 14:40
  • Runts are basically 'debris' from collisions. Those won't have a valid FCS. Maybe you have some bad equipment connected to this 3560, which is (for example) "forgetting" to pad small packets up to 64 bytes
    – mere3ortal
    Dec 6, 2017 at 16:30

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