The MIB used is RFC1213-MIB: http://www.alvestrand.no/objectid/126.96.36.199.188.8.131.52.1.15.html
For determining what packets are being dropped - try running a sniffer trace on the port and investigating the packet detail.
- ifInDiscards and ifOutDiscards represent the packets that are discarded
even though nothing prevented them from being passed (good packets that
were discarded to free up buffer space is an example)
- ifInUnknownProtos on the other hand represents the total number of
packets discarded because of an unknown or unsupported protocol
Calculations are as follows:
Spectrum uses the following OIDs or objects to show Interface Errors &
Discards for an interface.
Typically % Error will be the sum of above two OIDs, divided by Delta total
Similarly for % Discards , it will be sum of above three OIDs, divided by
Delta total packets .
Other than above OIDs Delta total packets is calculated based on following
Spectrum gets the value of above OIDs from device every ten seconds.
For example, at 0th second spectrum got values of above OIDs. These values
will be called as initial values. At 10th second, Spectrum will get again
more values of above OIDs. These values will be called as final values.
Now Spectrum will calculate the difference between the initial and final
value divided by delta time for each OIDs. To calculate % Error a formula
will look like as follows:-
% Error = ( ( delta_in_error_pkts + delta_out_error_pkts ) )
delta_in_error_pkts = final_value_ifInErrors initial_value_ifInErrors/
delta_out_error_pkts = final_value_ifOutErrors-
Similar way delta_total_pkts and % Discards will be calculated.
% Error and % Discards both are calculated in unit of 1/1,000,000th of a
So to calculate the "% Discard", typically formula will look like:-
% Discard = ( ( delta_ifInDiscards + delta_ifOutDiscards +
delta_ifInUnknownProtos ) )