The Hello reply payload is slightly different in Config and shard servers. Moreover, RSM expects to receive two separate replies from Config and shard servers. If we keep RSM to not know that the Config and shard servers merged it may conflict on host:port be the same for two servers. If we make the combined server to reply only once the internal SDAM protocol may require a non-compatible change.

We can also chose to keep the Config and shard servers on different ports to effectively separate them for the external observer. This will require to also create separate transports for them.