output discard

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Output discards are typically because there's more data to transmit than the receiver is capable of receiving. This could be down to non-linerate performance of the receiver or it could be due to speed adaption scenarios: e.g. data is received on a 10G port and must be sent out of a 1G port. You could identify which packets are lost by performing a packet capture close to the receiver (preferably using TAPs, but SPAN etc. may also be used) while simultaneously performing a packet capture closer to the source (i.e. on the 10G port if we continue with my previous example). If you compare the two, you'd be able to see which packets were sent but not received i.e. dropped by the switch/router.