Replicating from a Galera Cluster to a Slave is a bit tricky when it comes to server_id.
It is IMHO not properly documented if the server_id's on the Galera Cluster have to be all the same or different.
General MariaDB docu says: All server_id's in a Replication set-up must be different (without mentioning Galera).
Codership (Alex) told me: In MySQL Galera Cluster all server_id's must be the same in a Galera Cluster.
s9s says: They must be the same.
My experiments show me, that it works with all different.
But I would like to have a clear statement from the Software vendor...