Listening Port

Listening port parameter defines the listening port number that will be associated with this VSP .

Listening port usage is slightly different for TCP/IP and UDP/IP transport protocols :

For TCP/IP transport protocol this parameter defines a listening port on which incoming connections are to be accepted. Listening port is closed and irrelevant when the routing mode is "client", since in this mode incoming connections are not allowed in principle. Outgoing connections, when allowed (i.e. in the "client" and "server/client" routing modes), are established from an "automatic" port, so the listening port number has nothing to do with this.
For UDP/IP transport protocol the situation is as follows:
When the routing mode is "client" (incoming "connections" are not allowed) the listening port parameter is irrelevant and the VSP sends it own UDP datagrams from an "automatic" port.
When the routing mode is "server" or "client/server" the listening port parameter defines the listening port on which incoming UDP datagrams are accepted and is also used as the port from which outgoing UDP datagrams are sent.  

Listening port parameter is similar to the Port Number (PN) setting of the DS.