Ok thanks Brian. My instinct is to go with the default configuration - UDP. And I also understand from your comments why this configuration will be the best for most cases. The reason I ask is that about 3 years ago (before my time) it was reported that with our app (then using I think JBOSS 2.x) running Windows 2000 that when using UDP for JGroups some app servers would periodically hang while booting. This behavior was intermittent but when they hung there were error messages in the JGroups logs. I don't believe the root cause was ever discovered and so the decision was made to move to a TCP based configuration using the Gossip Router. This apparently stopped the 'hang during boot' behavior but we never understood the original problem. Since then we have read MS articles saying that Windows networking using multi-homed machines (which our app servers are) can have trouble when talking UDP. MS have a patch for this and we will try that. My question : have you guys at JBOSS ever heard of problem clustering JBOSS using UDP under Windows? If so, any details (especially resolution steps) you may have come across would be most welcome. We are now planning to start using full blown JBOSS clustering (session replication, tree cache) etc and I would like to be able to use UDP for all networking configurations. thx David View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=4150909#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...