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Solving latency problems with a local ISP

We have an ISDN connection to our local ISP, which is located 700 miles away. We can't figure out why we're getting high latency with our local ISP. When we remove our LAN from our network we get normal latency, but there is still some traffic flow through the BRI port of the router (Cisco 1700). If we connect the LAN to our router, then we get the high latency to our local ISP. We checked the ISDN line, the BRI port on the router, and the LAN for any viruses; all are OK. The utilization is also OK. Could you give me the solution for this issue?

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You must first determine what the traffic is. I would suggest a sniffer or some package that can decode the packets (place the sniffer on your outbound side). With the LAN disconnected, you may find that the traffic that is passing back and forth is address and route information. This is normal traffic.

There are a couple of things that could be causing issues when you LAN is attached. One being spyware - if you decode the packets you can find out what IP address is still broadcasting traffic. There could also be an error in someone's IP address, gateway, or subnet mask making them jump the router. I assume you are running a firewall, so it may be prudent to upgrade to the latest and greatest version. Your ISP should be able to provide you with the address of the communicating device if you don't have a sniffer. They should also be able to tell you what type of traffic (by IP port, anyway) so that you can more easily narrow this down.

If you don't have a sniffer and they can't or won't help you, try downloading one of the demo versions or a free one just to get you started. More than likely, you have a device or two that is being a bandwidth hog or perhaps your server is trying to communicate with your routers because it is routing.

As you don't say you are having any problems accessing websites, I am assuming that DNS is not a problem, but rather a single device or maybe a couple that are driving your latency up. If you try these things and are still having problems, feel free to send me your results and we can move on to another step.

This was first published in June 2005

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