The normal way to set up a Tomcat 6 cluster is to use multicast messages to manage cluster membership, with Tribes replication of session data (or deltas) to all known members. When multicasting is not supported in your network environment, static membership is an alternative you can try. It’s not as flexible, in that you cannot dynamically size a running cluster, but it has all the essential features such as session synchronization, balancing, stickiness and failover.

Assuming you are reasonably familiar with Apache httpd and Apache Tomcat, here’s the OS-neutral recipe.