GOAL
If you are registering a cluster and noticed that only some, (or none) of the nodes registered correctly to the cluster (see How to Verify if the Mule Cluster is Working Correctly) it is most probably caused because Multicast is not enabled in your network. Multicast will be used to announce/discover each cluster's member in the network and for initial negotiation, and enabling the multicast IP address 224.2.2.3 is listed as a prerequisite.
PROCEDURE
In order to verify if Multicast is working correctly in your network, you can use the following quick tcpdump/ping test.
For Mule 3.7.x onwards and Mule 4.x (Hazelcast based cluster)
In one node use tcpdump to listen in the ip 224.2.2.3, i.e:
tcpdump -ni eth0 host 224.2.2.3
where eth0 is the name of the used interface
And in the other node ping the IP 224.2.2.3
If the Multicast is working correctly then you should see packets arriving at the first node. Repeat this procedure in each node to verify that Multicast is OK in your network.
