OnixS C++ FIX Engine  3.25.0
API Documentation
Specifying Multiple Listen Ports

To accept (listen for) incoming connections on multiple ports, they should be specified simultaneously during the Engine initialization, using ListenPort Engine configuration setting.

If there is an incoming FIX connection on one of the configured listen ports, and the corresponding FIX session-acceptor is found without the specified listen port, or with the same specified listen ports as the 'active' listen port, the incoming FIX connection is accepted.

If the corresponding FIX session-acceptor is found, but the specified session listen port is not the same as the 'active' listen port, then the incoming FIX connection is terminated.