In an active-active setup, both servers go through normal TCP connection attempts which will include resolving the host IPs. Once this is done, since this is an active-active route, both servers will maintain the server information as they continue to try to re-establish the route. 

In the active-passive route setup, the active side will continue to try to use the known server information to create and/or reestablish a known route, but the passive side will not retain the other servers information when the route goes down and will recreate its state when the active member reconnects, including checks to make sure this new server does not create any loops in the existing know routed servers. 

During active-passive route establishment, the passive server does indeed perform a DNS lookup of the initiating server's host machine name. This is done in the process of determining whether creating the route would result in the formation of a cycle/loop between routes in a multi-hop zone (in which case the route would not be created). The hostname used in the lookup is obtained directly from the system on which the initiating server is running, rather than the config files. 

Example of logging and issue:

Active-passive route connected - actvei initiates connection but due to delay, connection times out

2020-11-22 00:41:25.703 WARNING: Route Warning: host of this name does not exist: xxxxxx
2020-11-22 00:41:25.703 Route 'NAME_hostname' accepted from host 'hostname' with zone 'default_1hop_zone:1hop'.
2020-11-22 00:41:25.703 Route configuration: Sending topics to server PRD_FX_APACSG1_GLOBAL_EXECUTION_server_name at tcp://hostname - processed connection initialization
2020-11-22 00:41:25.703 WARNING: Slow processing protocol message of type _INIT_CONNECTION_, lasted around 17000 milliseconds.
2020-11-22 00:41:25.703 Route 'route_name' disconnected, connection id=433786, reason: connection terminated
2020-11-22 00:41:25.704 WARNING: Connection timeouts delayed around 19 seconds.

In an active-pasive route setup, we need to make sure that the active side machine hostname, is able to be resolved from the standby machine side, otherwise, it will cause problems while establishing the route.

WARNING: Route Warning: host of this name does not exist: xxxxxx

Make sure the machine on the passive side of the route, is able to resolve the active side machine hostname