What do input discards on switch ports indicate and how can one track down their cause?

Question

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:

• Mismatched VLANs
• Frame did not need to be switched because the MAC was already on that port, or because it is the only port in the VLAN
• The ifInDiscard SNMP MIBs seem to indicate that buffer space can also be a reason

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

Answer 1

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.

Answer 2

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.