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Persistent UDP checksum error in SNMP messages

Hi, I'm having an inconvenient during the monitoring of a Cisco 2800 Router, related to a udp checksum error in snmp messages sent from one of the WUG server PCs (I'm using Wireshark to analize this). In that computer, I had to install a previous version of a comercial monitoring program because that computer  was Windows XP. All the snmp traffic generated from this computer didn't have problems, in  Wireshark all the layers of each snmp message didn't have errors. But I needed to run the latest version of the monitoring program, so I installed Windows 7 in a external hard disk and then I could install it.

If I do snmp requests with that program (and also, for minor snmp requests, an open source executable program too) using the external hard disk, I can retrieve de objects and data I want from the router, but curiously the snmp messages sent from the monitoring computer ( have a checksum error (they appear in black color) and inside of the message layers you can realize that the problem is the network layer, because of a  wrong checksum, always with a value of 0x00. The snmp response messages, sent from the monitored router ( to the monitoring pc server, don't have errors of any kind (they appear with the normal light blue color).



I tried connecting the external hard drive to another computer and I have the same situation, the snmp messages that leaves the computer have always the same udp checksum error. This only happens when I poll monitored devices using the external hard drive with Windows 7, and no when I do the requests using the computer's hard disk that has installed Windows XP with the previous version of the monitoring program.

Do you have any idea why using the external hard drive with Windows 7 causes this checksum errors? What is the reason and how can I solve this?

Thanks in advance.

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