This Integer32 is described as the UDP port that the server is listening to, i.e. the IKE service port for this virtual router. An admin would typically leave this at the standard IKE port but could change it in scenarios requiring a non-standard port for NAT traversal or firewall policy reasons. For example, an admin working around a strict upstream firewall that blocks the default IKE port might reconfigure this to an alternate port both ends agree to use.