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The conditions under which a switch will discard outbound packets on particular ports are fairly well established in many basic training materials, including those for CCNA. However, input discards are typically less well-documented. Some reasons I've found include:

Are there any other common reasons? How would one decide between these possibilities?

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    What vendor / platform. Please give us the actual screenshot where you see discards – Mike Pennington May 6 '14 at 8:00
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    The switch which triggered this question was an HP 5120 running Comware 5.20, but it was picked up in our monitoring system, not in the switch itself. The monitoring system is just using the ifInDiscard data from the SNMP MIB. – Paul Gear May 7 '14 at 1:29
  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Aug 9 '17 at 15:35
  • I consider this genuinely unanswered, esp. with respect to how one decides between the possible causes. – Paul Gear Aug 10 '17 at 0:12
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Unknown VLANs are the primary cause of Input Discards (ifInDiscards) in my environment; usually from inappropriate VLANs spanning a trunk port. Depending on the services active in the VLAN in question, those counters can increase exponentially over short periods of time.

Keep in mind that Input Discards are the result of valid frames being dropped due to an internal forwarding issue. Another thing to note: Input Discards encompass a drastically smaller amount of issues, most everything else results in an interface error.

  • The second part of my question is the more critical in my judgement. You say unknown VLANs are the primary cause in your environment; how did you determine this? The algorithms for outbound discards are taught as a fundamental part of switching training, and they are eseentially deterministic, but advice about input discards seems to involve a lot of "it might be this, it might be that, test X and see if they go away". – Paul Gear May 7 '14 at 1:31
  • Input Discards/Errors are fairly generic counters; and that is by design. When you glance over an interface, you're already bombarded by quite a bit of information. It will take a bit of digging to get to the root cause. The best place to look for these types of incrementing counters on a Cisco platform is under the show controller ethernet-controller section. I'm not sure what the equivalent is on HP's platform. – Ryan Foley May 7 '14 at 7:35
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One other reason is that STP topology changes frequently. This usually happens when you have a flapping link (up and down) somewhere in a Layer 2 switched network that has either RSTP or MSTP enables, or any other Version of SP with "portfast" configured.

What happens that, whenever the flapping link goes down, a BPDU is sent to update all the switches in the network about the new event, which basically leads to a topology change. When this occurs, it could cause some switches to discard a few packets. If the link is coming up and down repeatedly, a lot of frames will be discarded.

Note: I had this issue many times with switches that were non-Cisco made. I never had this problem with Cisco devices.

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