The -u option will have multicast address, this is not a 'normal' IP address (often multicast is blocked by router/switches)
You might separate a cluster with this options:
-Djboss.partition.name=XXX -Djboss.partition.udpGroup=126.96.36.199 (same as -u) -Djboss.hapartition.mcast_port=39993
With the partition.name it is ensured that two nodes are in a separate cluster if the name is different even if the multicast address is the same (might be a performance issue).
If the mcast is blocked between two nodes you become only in trouble if you WANT to have a cluster.
Hi Wolf-Dieter Fink!
As you said, jgroup multicast ip is not a normal IP addr, so is it a resource of my system? Why you set the param: -Djboss.partition.udpGroup=188.8.131.52 ?? Why this IP: 184.108.40.206? This IP has to satisfy what conditions?
Multicast addresses are a special part of IP address space (220.127.116.11 to 18.104.22.168) see e.g. wikipedia http://en.wikipedia.org/wiki/Multicast_address
My address is only an example.
You have to ensure that all instance of JBoss that have to build a cluster use the same address/port and partition name.
If another machine know (configure the same) jgroup mcast address/port and partition name, it join the cluster legal, it works as designed.
You can't configure the same multicast and say 'don't allow 192.168.1.1 to join' !
So if you do not want to build a cluster use a different partition name and, to avoid unnecessary traffic, a different multicast address/port.