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What is the goal for the modelling? Just standard type support, or more advanced collection, do you want to collect some performance data about how a protocol is operating, or verify the number of sessions a firewall is running.
Sometimes you can find the source MIB in the documentation or a whitepaper, but sometimes it is very difficult to determine where the needed data is stored. If possible ask a product expert who is familiar with that specific product.
Device Instrumentation
Many times, people want to graph CPU and Memory, but not all devices support the collection of this information, you can only ask NMIS to collect something which the device has the instrumentation for, the MIBs should tell you if it is possible.
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Now you know what you want to model, you will need to decide to use the system or systemHealth techniques for having NMIS collect and store the data, you will also want to decide if you want any custom alarms configured.