If you use Apache httpd name-based virtual hosts on the server with multiple IPs (without explicit configuration parameter ServerAdvertise On http://hostname:port), advertise does not work and sends 0.0.0.0 IP address to the back-end. This is problem when you have back-end server on the physically another machine.
Module should use ServerName from the VirtualHost configuration value in this case (or this case should be documented).
Refer to official Apache 2.2 documentation for more information on Name-based Virtual Host Support
Snip from mod_cluster docs:
> ServerAdvertise On http://hostname:port: Tell the hostname and port to use. Only needed if the VirtualHost is not
> defined correctly or when VirtualHost is not used.