SnapMirror uses the typical socket/bind/listen/accept sequence on a TCP socket.
Synchronous SnapMirror requires additional TCP ports to be open. The source storage system listens on TCP ports 10566 and 10569. The destination storage system listens on TCP ports 10565, 10567, and 10568. Therefore, you should ensure that the firewall allows a range of TCP ports from 10565 to 10569.