I have a small WAN -- Windows 95 and 98 PCs using terminal emulation through terminal servers to a Unixware server....
Remote sites are also PCs using terminal emulation through Terminal Servers. The remote links are ISDN with pairs of 3Com311s routers. I introduced TCP/IP, and the PCs now continually activate and drop the links. It seems to be making a call every few seconds, with the telephone bills going through the roof. I am told this is a Microsoft IP broadcast issue or that I need to unlink NetBIOS from IP (the box is grayed out). Can you help? Microsoft NetBIOS Browser broadcasts are most likely to cause this effect. I have answered some other questions on this, you might want to look at previous questions and answers page to get a better understanding.
You have two choices to fix the problem. One is to remove NetBIOS from the protocol stack of all the machines in your network and then hope that you have got them all. In my experience, this is usually a bad idea but people who understand operating systems and not routers often take this path.
The usual (and best practice) method is to configure your router to allow the dialup to happen only when certain types of traffic need to cross the ISDN line. In your case, the only traffic over the dialup is terminal server, so you might want to block all other traffic from entering the link to make sure that there is plenty of bandwidth for the terminal server flows.
I don't have any experience with the 3Com routers you mention. I suggest that you locate someone who has done this before (and can prove it) to help you with this. Otherwise, feel free to ask more questions and I will try my best to help.
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