Q

Troubleshooting your network

In this Q&A, our expert, Lindi Horton, helps with troubleshooting a network that has problems staying up in the evening.

I have problem in my networking strategies.

After 6:30 PM my network goes down. We are not able to get the mapped drives on local machines and we cannot save anything on our server. We have a Dell Poweredge 2800 server with Microsoft 2003 as our operating system. We have recently put Pix 515 firewall on the switch (D-Link DES 3550). In the daytime it works very smoothly, but by evening it chokes. We have scanned all the machines with Trend Micro for viruses and found that there are none. Please suggest something I can do.

This is an interesting problem. As with all troubleshooting problems of this magnitude and impact, the creation of a true problem statement and investigation into that is essential to understanding how to resolve the issue.

To troubleshoot this issue, I would suggest loading up some packet analyzers to try to isolate and find the problem. For example, I would have network monitoring captures on the servers going on during the time that you have issues. This would help you understand if there are packets actually arriving at the servers and whether or not the servers are responding to those requests. This would allow you to see if it is the network or the server that is experiencing the problem. It would also show you if the server is being overloaded with backup or other types of traffic that might cause it to stop responding to other types of requests. You would see a lot of RST bits set if the servers are resetting sessions. This would be the first start. Alternatively, you can start looking at either getting a tap or doing a SPAN (port mirror) to determine what the traffic patterns look like. NetFlow and other IP accounting technology also would help determine where traffic is going and what protocols are consuming the traffic to impede response time.

This was first published in October 2006
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