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Trends in Technology: The Details of SNMP
First let me introduce a couple of acronyms: MIB and OID. MIB stands for management information base. This is just a small file (which you can copy and paste to a text editor if you want to see what it really looks like) that (conceptually) is like a map that tells an SNMP manager what variables are available to be read and where they are. (If you want to get in to more details about how a MIB is constructed then check out the IETF RFC 1157). OID stands for object identifier. The format of the OID is a string of integers separated by decimals. The MIB is basically a structured collection of OIDs.
If you have a device that supports SNMP and will operate as an SNMP agent, the first thing you will need to do is to obtain the MIB from the device manufacturer. Usually that comes with the device when it's new out of the box (on a CD). Sometimes you can obtain it by browsing the device itself, or by obtaining it from the support Web page of the device. If the device supports SNMP then the MIB will be available somehow.
Take for example the case in which two Relios talk to one another; one being a manager, one being the agent. As the user you would do three things: Find the MIB for Relio (downloads from the unit), and make it available for the Scripteasy software (upload to a location accessible by Scripteasy). In the script on the manager, identify the IP address of the agent, and specify the MIB to be read when querying the agent.
Scripteasy is a scripting program done completely with symbols -- all laid out visually. It's particularly easy to see the OIDs with Scripteasy, so it makes for a good example.
In Figure 3, you see part a small part of a script running on the agent. I've highlighted a symbol that corresponds to a temperature probe in our transmitter space. On the right, you see I've highlighted the OID. Figure 4 shows the script on the manager. The symbol you see corresponds to SNMP get (in this case, getting an analog reading from the agent). You see that I've highlighted the OID again; obviously it's the OID from the agent (note the OID is the same). You can also see where I referred to the agent in the link to section (right above the OID).
What you will then have is a remote, real-time meter on the manager Relio that corresponds to the same meter over on the agent Relio. Making dynamic graphs with information gleaned by way of get request is a common function with SNMP. I should also note that this same agent will respond to multiple managers; I could paste this same analog temperature meter in multiple places, all at the same time.
-- continued on page 3
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