The MIB file is a contract between a MIB browser and SNMP agent. SNMP agent implements MIB objects declared in the MIB. MIB browser parses MIB file and learns what objects are supported by SNMP agent. Usually MIBs are tested in some environment. However switch to a new MIB browser may expose unpredictable behavior for MIBs with errors and/or warnings. Unfortunately many even published MIBs have errors and warnings. Ideally our MIB should not have any errors or warnings at the highest level 6.

There are 4 known online free MIB validators. Running the MIB validation against multiple different validators may help to fix the MIB because validation messages in some cases are very cryptic. Online validators are using different backend engines and may produce different results.

libsmi (cgi)
libsmi (html)
This online validator is using libsmi software package as its backend. The validator allows uploading single MIB and one import MIB file.

SimpleWeb
This online validator is using libsmi software package as its backend. The validator allows uploading single MIB or pointing to a MIB's URL.

Muonics, Inc.
This online validator is using Muonics MIB Smithy SDK as its backend. The validator allows uploading single MIB.

AgentPP
This online validator is using JASMI-2 as its backend. The validator allows uploading single MIB or a zip file with multiple MIBs.

See also:
17 Common MIB Design Errors
MIB Design Dos and Dont's