iptables or any other firewall cannot open a port by itself - only an application that uses that port can do that.
What a firewall can do is unblock a port to allow connections to it. But without an application that either listens for incoming connections or opens an outgoing connection using that port, an unblocked port will still be closed.
A blocked port will typically drop incoming packets, ignoring them completely. For the person attempting to connect to a blocked port, this typically shows as a "connection timed out" error after waiting a while.
An unblocked but closed port will typically actively reject incoming connections: the person attempting to connect will get a "connection refused" error message pretty much immediately. Depending on network protocol used, this rejection can usually happen in one of two ways: for TCP, a TCP Reset packet is usually sent. The other option is to send an ICMP error message packet, which is the norm with UDP and may also be used with TCP.
Port 1701 is typically used by a L2TP VPN protocol, which is often used together with IPsec.
On OpenSuSE, IPsec is provided by package
strongswan and L2TP by
xl2tpd. You may also want the
yast2-vpn package to allow for easy configuration (either client or server side) through YaST. You would need to install and configure these to suit your requirements, as they won't be enabled by default.
It is unclear whether you plan to use your OpenSuSE as a VPN server (to accept incoming VPN connections from the Windows server) or as a VPN client (to make an outgoing VPN connection to the Windows server); in either case, these links to OpenSuSE's own documentation may be useful: