InterfaceErrorsExamples

From GEANT2-JRA1 Wiki

This section shows basic examples of Input Errors and Output Drops acquired on physical interfaces. The purpose is to explain the meaning of these quantities and to report common situations occurring on the circuits. All the examples shown are related to actual access circuits corresponding to GARR users.

The RFC definitions of these quantities are the following ones (from RFC 1213):

ifInErrors:

"The number of inbound packets that contained errors preventing them from being deliverable to a higher-layer protocol."

ifOutDiscards:

"The number of outbound packets which were chosen to be discarded even though no errors had been detected to prevent their being transmitted. One possible reason for discarding such a packet could be to free up buffer space."

These are general categories while the specific physical meaning is vendor dependent.

How to read the scale on the graphs

By default, MRTG is configured to poll every 5 minutes. On the vertical axis errors per second are reported: this value is obtained as the ratio of the number of counted errors in a 5 minutes interval to 300 seconds. For example, if a counter reports an increase of 10 errors in a 5 minute interval, the value shown on the graph is 33.33 merrs/sec (0.03 errs/sec). The value for a meaningful threshold in the observed counts depends of course on the context. For example, an unofficial rule for the GARR NOC is to neglect occasional spikes below 50 merrs/sec (15 errors over an interval of 5 minutes) Furthermore, to understand the size of a problem, we need to take in to account not only the rate but also the integral in time.