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SNMP Around the Station
SNMP in action
Let's look at a MIB browser. An MIB is basically a small text file (you can paste it in to notepad if you want to see what one looks like) that the manager reads to find information in the agent. Conceptually, the MIB is configured like a tree; as you go up the trunk and out the branches, the information becomes more specific, until you reach a leaf. The leaf is the end and represents a very specific piece of information that you want the manager to be able to see. The leaf is found by the manager by its reading of the OIDs (object identifiers) that are present in the MIB.
The browser I use is made by iReasoning Network. Go to ireasoning.com/mibbrowser.shtml and note in the right hand column a link to download free personal edition. Download the MIB browser from there.
For this demonstration we're going to use a Broadcast Devices DPS-100D in-line wattmeter. (I've chosen this device because the MIB is simple and illustrates the point well.) After you launched the MIB browser, download the MIB for this device. Go to broadcast-devices.com/images/BDI%20MIB%20FILES.zip, unzip the files and save them somewhere convenient. The final step in setting this up is to run the MIB browser, then select File > Load MIBs. Then, with the open window, find the Broadcast Devices MIB files you just unzipped. First click BDI DPS-100D MIBS\BDI-Enterprise-MIB-SMIv2.mib and click open, then click BDI DPS-100D MIBS\BDI-DPS100D-Product-MIB-SMIv2.mib and click open. You need both.
With all this done, you can type in an IP address in the window in the upper lefthand corner of the MIB browser. The address for our demonstration unit is 220.127.116.11. Then select Operations > Walk. You should then see the result table populated.
This remote device has been left open for purposes of this demonstration; note however that there are basic security features with SNMP (version 1) namely the Read Community and the Write Community. (If you click on Advanced Features you'll see where to enter those.) These are basically just passwords, and typically the Read Community is the word
Notice the results table has four columns: name/OID, value, type, and IP:port. As I browse this MIB I noted a couple of things I wanted to be able to read: the forward and reflected power (for example) and the internal temperature along with the external temperature. Looking in the next column, I see their values (what they were when the walk was executed); and I see the OID type. Here is where this gets useful: Notice that if you highlight any one of the lines in the results table that its OID (Object Identifier) shows up in the OID window, which is near the top of the page, above the results table. The OID (as you can see) is a series of numbers separated by decimal points: One example would be the forward power OID: <18.104.22.168.4.1.37622.214.171.124.126.96.36.199.0>. Note there is a one-to-one correspondence between this OID and the information leaf you want to read.
When you configure your SNMP manager, the OID and the type are the information the browser needs to be able to read. The value will be read every time the agent is queried by the manager (Get). You can do this manually by highlighting the OID you want (forward power in this case) and then using the operations window (upper righthand corner) and choosing Get, you'll see the MIB browser return the value again. Of course your SNMP manager will automate this function.
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