Second, what are the steps you followed to add the device ? Are you using node seeds (ie nnmloadseeds.ovpl)?. You can also use the nnmnoderediscover.ovpl script to add nodes to the NNMi discovery queue. See the nnmnoderediscover.ovpl reference page, or the UNIX manpage, for more information
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Did you try to seed it like so nnmloadseeds.ovpl -n <Fortinet device> on some devices auto discovery may not work (see the NNMi release notes) , even if the device is not on the device support list NNMi must discover any device that respond to SNMP and/or ICMP if you properly configured your discovery rules.
If it's taking an hour to complete snmpwalk, and you've got devices in production showing "No SNMP access", then I'd be tempted to push it back to Fortinet, and ask them why it takes so long to get SNMP responses.
Does it just slowly keep returning results, or are there certain OIDs that take a long time to respond?
Assumed that you have access to device and is using correct community, I suggest to you insert the mib for this device (that follows attached with documentation) and do the properly configuration if you need specific data from fortinet.
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