There is no interface between SGWs. All the signalling within the LTE network is controlled by MME. During the attach procedure, SGW and PGW selection is done by MME. As part UE mobility, in some scenarios SGW gets changed for an UE and this time again MME takes decision to select appropriate SGW. In case of SGW relocation, MME initiates handover signalling towards the selected SGW. Old SGW and the new SGW does not talk to each other directly.