AS/400 has attached the following RFC:
Frame Relay (RFC1315)
All of these, can be downloaded from IANA website.
If private MIB from IBM become available to download, you can use Universal Device Poller. Check this video:
... I am not getting the CPU and/or memory information on a device
... Why does NPM list so little IBM TS3200 equipment information?
... SNMP is working fine but is not getting some CPU or memory information?
Example devices: Fortinet, Enterasys, Extreme, Bluecoat, AIX, AS/400 ("IBM iSeries"), AC6005, Avaya, Aruba and others.