I understand the basic use of SNMP where you manage and monitor switch, routers,etc. features using IP network. I also understand there are MIBs for SNMP based on RFCs like 2981, 3289, 3376, etc. However, recently I heard from couple of customers talking about private MIBs and it seemed like they were really interested in it.
Can somebody explain to me what are private MIBs? Why would somebody be interested in it?
Are there private MIBs equivalent for MIBs based on RFCs? For example, is there something like a private MIB for RFC 2981 MIB?
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.
Private MIBs refer to MIBs associated with a specific Private Enterprise Number which starts at the following prefix: iso.org.dod.internet.private.enterprise (18.104.22.168.4.1)
These numbers are assigned by IANA and are a namespace for an organization to store their custom OIDs/MIBs. You can see the full list of PENs here. For example, all of Cicso's proprietary OIDs would be under 22.214.171.124.4.1.9.
While private is in the name, they're not, but could be, private. It's just a namespace. If you're going to be creating your own OIDs you should register a PEN to avoid collisions with another vendor.