The MOBIKE IKEv2 extension (RFC 4555) allows an initiator to change its network attachement point (e.g. roam to an other interface/address).
strongSwan implements MOBIKE by watching interfaces, addresses and routes. If the configuration changes, route lookups are done to find a better path than the current one and, if necessary, the path is changed using a MOBIKE update (UPDATE_SA_ADDRESS).
strongSwan is running the MOBIKE protocol per default. MOBIKE can be disabled on a per-connection basis, though, by adding the parameter mobike=no to the corresponding connection definition in ipsec.conf. Please be aware that with MOBIKE enabled, strongSwan will switch to UDP port 4500 starting with the IKE_AUTH request, which includes a MOBIKE_SUPPORTED notification, even if no NAT has been detected. Thus make sure to open the NAT-traversal port UDP/4500 on any firewalls en route.