Cisco Switch Port Type – How to Recognize via SNMP

Private MIBs are usually vendor specific. They contain additional information for their specific equiment which cannot be provided through RFC-defined MIBs. This way the vendor can provide more information.

To answer your second question: sometimes vendors provide both: the RFC-defined MIB contains all that can be contained there (so most software using these MIBs can use it) and the private MIB contains the additional data (and sometimes the RFC-defined data as well).

Basically it's all up to the vendor to put in the MIB what he deems useful, but it's very useful to use the RFC-defined MIBs where possible so many tools can use it out of the box.

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.