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AuthorForum: PowerSNMP for ActiveX
Topic: Get and GetNext return different values for same OID?
ssmiley

From: Portland, ME USA
Posts: 2
Member Since: 04/03/09
posted April 3, 2009 7:28 PM

I'm confused by what appears to be a strange SNMP behavior.

I'm trying to query a rack fan unit that reports the status of each fan in a table. 

If I do a GET request using the table OID + index I get the value "1" back (which is incorrect).

If I do a GETNEXT request using the table OID, I get the value "3" back (which is correct).

When I compare the Variable.OID from the GETNEXT request it's exactly the same as the OID I used on a regular GET Request.

So my question is, if both methods are using the exact same OID, why do they return different values?

Here are the specifics of the problem I'm seeing:
GET Request:
============
Query OID:   1.3.6.1.4.1.318.1.1.14.4.1.1.6.1.1
Variable.OID = 1.3.6.1.4.1.318.1.1.14.4.1.1.6.1.1
Varaible.Value: 1

GETNEXT Request:
Query OID:   1.3.6.1.4.1.318.1.1.14.4.1.1.6
Variable.OID = 1.3.6.1.4.1.318.1.1.14.4.1.1.6.1.1
Varaible.Value: 3
Jason Farrar (Admin)

From: USA
Posts: 223
Member Since: 07/24/07

Extra Support Options
Custom Application Development

posted April 6, 2009 2:02 PM

It could be that there is a bug on the Agent side were it is responding differently to a direct Get message in regards to tables versus a GetNext Message. The real test would be to do a get on that OID with another SNMP utility and see what it responds with. Please let us know what you find.

~Jason
ssmiley

From: Portland, ME USA
Posts: 2
Member Since: 04/03/09
posted April 6, 2009 2:08 PM

After posting my original message I went and downloaded the trial version of the commercial SNMP application OIDView.

The direct query tool in OIDView exhibits the exact same behavior as what I'm seeing with the Dart SNMP.

So it would appear that the Dart library is behaving correctly, even if that behavior is a bit odd.

Is there something in the underlying SNMP Spec that indicates a behavioral difference between GET and GETNET?
Jason Farrar (Admin)

From: USA
Posts: 223
Member Since: 07/24/07

Extra Support Options
Custom Application Development

posted April 8, 2009 1:32 PM

Well the difference is that a GET message should get the OID that was specified while the GETNEXT Message gets the next OID in sequence from the specified OID. With a GETNEXT message the agent calculates everything that it will send back. If you are seeing this behavior with another utility then it sounds like this could be a bug in the agent software itself.
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