Should the servlets and EJB replication be within the same multicast subnet?
Yes, this is the recommendation. Even though there are separate methodologies to defining “islands” for servlets and EJBs, it is recommended that the constituent member processes be the same. This is especially important if the servlets use the services provided by the EJBs. This will ensure that both the servlet and EJB states is available in the same set. Un-availability of either one of these otherwise could cause failure.