The roadwarriors carol and dave set up a connection each to gateway moon. The strong mutual authentication is based on EAP-PEAP only (without a separate IKEv2 authentication) with the gateway being authenticated by a server certificate during the EAP-TLS tunnel setup (phase1 of EAP-PEAP). This tunnel protects the ensuing weak client authentication based on EAP-MD5 (phase2 of EAP-PEAP).
With the setting charon.plugins.eap-peap.phase2_piggyback = yes the server moon initiates phase2 of the EAP-PEAP protocol by piggybacking a tunneled EAP Identity request right onto the TLS Finished message. Client carol presents the correct MD5 password and succeeds whereas client dave chooses the wrong password and fails.