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I have a production server that is responsible for controlling a factory. The server runs a number of control applications and a SQL server.

The problem I have is that one of the applications that communicates with a PLC is reporting communication problems at seemingly random intervals.

Using resource monitor, I have noticed the network activity drops sharply whenever this problem occurs. My VNC connection is not interrupted and the server responds to pings from other computers during the event/blip, however, other computers on the network which run applications connected to the SQL server are freezing till the network traffic restores.

Screenshot of the resource monitor network graphs at the time of a blip. the first arrow is when we start experiencing communication problems and the second arrow is when things return to normal:

screenshot of the resource monitor network graphs at the time of a blip

I have analysed the SQL server at these times and there are no resource waits and the number of batches processed per second are also low. I also did a trace on the SQL server at the time of a blip but this did not reveal anything significant.

At the moment just before the network activity drops there are no other indications this is going to happen. The CPU is low and memory is usage remains at about 70%.

Could this be being caused by external factors affecting the network or maybe something wrong with the network card?

Edit (additional information):

This is a performance monitor for packets sent and received at the time of the blip:

enter image description here

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    Could this be being caused by external factors affecting the network or maybe something wrong with the network card? - Yes it could. Do you have other systems connected to the same switch? If so, run resource monitor on one of those hosts and see if there's a corresponding dip. If there is then I would start taking a look at the switch and the network traffic. – joeqwerty Feb 18 '14 at 16:44
  • I ran the resource monitor on a different PC on the network. I didn't notice any massive change when the blib occurs, but there isn't that much traffic so I'm not sure if that's really telling me anything. – SausageFingers Feb 19 '14 at 16:31
  • This server is responsible for a lot of the traffic on the network. If the server has a problem and stops sending/receiving packets for what ever reason then the general network traffic as observed from another computer would also dip, so I'm not sure if focusing on the network switch is a bit of a red-herring. – SausageFingers Feb 20 '14 at 8:50
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Try installing Wireshark (www.wireshark.org) you'll see what happens to each packet... I had a similar issue with my Exchange 2010 Server and after analysing the packets, i discovered that the issue was with IP Fragmentation which i resolved by reducing the MTU of the server's NIC. So, that might be a reason for the packet drops.

Check this link: http://www.networkworld.com/community/blog/mtu-size-issues

  • I ran wireshark on a different PC on the network. There appear to be a dip in traffic that corresponds to the blip but I cant be that sure. I have never used this tool before so I'm not sure what to filter out or look for. Also, the adapter MTU is currently set to 1500, which I think is probably correct. – SausageFingers Feb 19 '14 at 16:34
  • You can pause the capturing and then check each packet... The coloured packets (black, red mainly) show the faulty packets. You can expand the info on each packet to get more details on the issue. What i did was to google each error i observed – Babatunde Feb 19 '14 at 16:39
  • I took a trace using wireshark but couldn't find anything obvious that was causing the problem. I have an opportunity to install the latest NIC driver soon. I am also going to try disabling Large Send Offload and TCP Checksum offload to see if that makes any difference. – SausageFingers Mar 26 '14 at 10:48

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