It seems the M3UA Heartbeat is enabled only on signalling gateway and it is disabled on MSC - core network.
MSC never sends M3UA Heartbeat to signalling gateway and only gateway sends M3UA Heartbeat to MSC and MSC only responds with M3UA Heartbeat_Ack.
It seems one of the IPSP =1 went down due to M3UA Heartbeat failure.
Signalling gateway sends SCCP CR on IPSP=2 but MSC keeps assuming IPSP=1 is Active and keeps selecting the IPSP=1 link for SCCP CC and signalling gateway keeps dropping the SCCP CC as Unexpected message on IPSP=1.
Is this because SCCP CC is coming on a different( IPSP =1 )instance than the one on which SCCP CR ( IPSP =2 )was sent.Hence M3UA layer at signalling gateway is dropping the SCCP CC and is sending M3UA ERR response to MSC ?
Is MSC misbehaving here by sending SCCP CC on a different IPSP instance =1 ?
Or is it because IPSP =1 is down locally at signalling gateway due to M3UA HBEAT loss and the peer node MSC is not aware of this.Hence it keeps selecting IPSP=1 for sending SCCP CC ?
Both IPSP=1 and IPSP=2( SCTP Multihoming enabled) belong to Iu CS Interface :1 which is connected to MSC.