Unshared State

When configuring a fault tolerant pair that does not share state, you must ensure that both servers use identical configurations.

This is especially important for these configuration settings:

Destinations

Both servers must support the same destinations.
Routes

Messages must be able to arrive at the endpoints, using equivalent or identical routes across servers.
Access Control

Access control must be setup identically in both servers, so that the users.conf, groups.conf, and acl.conf file settings match.
SSL

When SSL is deployed, both servers must use the same certificate(s).