Config Server Functionality
Clusters’ JMS implementations require ConfigServers. It serves as the coordinator for other servers joining the cluster. It just needs to be running for other servers to join the cluster. This is true for either an application server cluster or a mediation server cluster.
When the config server has an error and doesn't work, depending on the error and how early it occurs in the startup sequence it may be prevent other servers from joining the cluster.
If there is some sort of fatal hardware issue with the ConfigServer another host can always be defined as the ConfigServer, making clustering possible, and allowing other cluster members to join that cluster.