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NPM reports the numbers found when it queries the SNMP agent on a device. Discarded traffic, whether it is intentional or not, is recorded by the following MIBS: ifInDiscard, ifOutDiscard, ifInErrors, ifOutErrors. Since these MIBs are Counters that Increment, our application will take an Initial poll and record this number in the Service which is not stored into the Database, then on the Next poll it will take the delta between the newest poll minus the last poll and record the information into the InterfaceErrors_Detail Table.
If you do not wish to monitor Errors and Discard, go into Manage Nodes, select the Node and select List Resources. Under Each Interface you can uncheck Interface Error Statistics, or you can go under the Top XX Errors & Discards, select Edit and enter in Vendor <> 'Cisco'
NPM is designed to report all data for Networking teams to see at a glance all Network related issues at a glance to help see and know what device or where in the network that that there are potential issues. Packets denied through firewalls such as an ASA will increment on the Discards columns, and Cisco APs report data transmission issues on Discards as well. Physical line issues will be shown in the Errors column.
You will need to contact your Device’s Vendor to understand what the Errors or Discards are, since Vendors represent Discarded Packets differently. Here is a paper from Cisco about their Discards:
If you are seeing a lot of Discards or Errors from a Cisco Device, here is a document on how to Troubleshoot Switch Port and Interface Problems