We have 3 Orion environments, 2 of which are in our Production environment and running on Win 2008 R2 servers and 1 is a Dev box for testing and general tomfoolery that is running on a Win 2012 box. All three had a Mibs.cfg file size of 1.3 GB. In case you don't know, this file is the SNMP OID Database SolarWinds uses for the Universal Device Poller application and the Manage Pollers page for polling CPU, Memory, Node Details, and custom SNMP polling. So when you are using their MIB Browser feature you are actually browsing this file.
Per the documentation it says that the latest MIBs.cfg file at the time of release should contained in each product upgrade. So typically just upgrading Orion should automatically update the Orion MIB database. Since NPM 12.0.1 was packaged and released on September 28, 2016, our MIBs.cfg file should have been current up to that time. Now, I'm not sure how much has been added to the Orion MIB database between Sep 28th and now, but the newest version of the MIBs.cfg file is nearly 1.9 GB in size once unzipped... That's almost a 50% increase in size (probably closer to 40%, but still) in just over a month's time... My guess is they didn't package up the newest MIBs.cfg file in the NPM 12 release. I would almost guess that they may have even put a substantially older MIBs.cfg file in there because an extra 600 MB worth of MIBs is a lot. That file has been around for years and years, so even if it gained 600 MB in 6 months I would be surprised.
In case you don't know this as well, SolarWinds updates the MIBs.cfg database file every two weeks based on customer support tickets, so instead of waiting for a product upgrade you can keep your MIB database up to date yourself. Usually this isn't necessary unless you have a node you're trying to apply custom SNMP monitoring to and Orion doesn't know about the OID's you are trying to poll. The file can be downloaded from your Customer Portal under "Helpful Links". You then simply unzip the downloaded file and replace the old file, located at "C:\ProgramData\Solarwinds\MIBs.cfg", with the new one. I recommend renaming the old one first to make a backup of it (I simply change its name to "MIBs.cfg.OLD_todaysDateHere") just in case. Then you restart all Orion Services and you should be good to go. If you have more than one polling engine then you will need to do this on your Primary Polling Engine and all Additional Polling Engines (I'm not sure about your Additional Web Server if you have one of those, but my guess is if that file is found in that spot then you should probably replace it).
CONCLUSION: The moral of this post is that you should check your MIBs.cfg file and see if it is anywhere close to the size of the new one (1,896 MB to be exact). I also recommend going into the MIB browser on your web console or into the UnDP application on your server and browsing the MIB library there to see if it works. I was getting an error in UnDP and just a blank page in the web's MIB Browser until I replaced the file and restarted the services. And yes, I did try restarting the services first before replacing the file and that didn't help. Only once I replaced the file and restarted the services did it help.
Also, please reply if you too notice this. I'd be curious to know how prevalent this issue is or isn't.