Method for a mobility management entity, MME (MME_1) in a pool of MMEs (MME_1; MME_2; MME_3) being adapted for communicating with at least one other MME (MME_2) in the pool of MMEs, the MME (MME_1) moreover being adapted for communicating with a serving gateway, SGW (S_GW), the MME (MME_1) managing the mobility of user entities for establishing packet data communication between the serving gateway and user entities, UEs, via at least a base station (eNB); a respective UE having a respective International Mobile Subscription Identity number, IMSI; the MME (MME_1) of the pool being adapted for storing at least a user entity context (UE_CTXT) for which a corresponding user entity (UE_2) is registered; a sub-group of the MMEs, denoted back-up MMEs, in the pool is adapted to hold duplicate user entity contexts (DPL_CTXT_UE1, 3), the method comprising the steps of - establishing a user entity, UE, context for a user entity such that the UE is registered in the MME in which the UE context is established, - appointing another MME in the pool as back-up MME for holding a duplicate UE context of the user entity; - informing (406, 018) the SGW of the back-up MME in which the duplicate UE context is held, or is to be held; and - transmitting (408, 022) a duplicate UE context to the appointed back-up MME, characterized in that the step of appointing is performed by - uniquely identifying a particular back-up MME in the pool according to a predetermined function using as input parameters the IMSI of the UE, the number of back-up MMEs in the pool and an order number of the MME in which the given UE context is registered, the function moreover securing that the back-up MME is chosen to differ from the MME in which the UE is registered. the MME (MME_1) moreover being adapted for communicating with a serving gateway, SGW (S_GW), the MME (MME_1) managing the mobility of user entities for establishing packet data communication between the serving gateway and user entities, UEs, via at least a base station (eNB); a respective UE having a respective International Mobile Subscription Identity number, IMSI; the MME (MME_1) of the pool being adapted for storing at least a user entity context (UE_CTXT) for which a corresponding user entity (UE_2) is registered; a sub-group of the MMEs, denoted back-up MMEs, in the pool is adapted to hold duplicate user entity contexts (DPL_CTXT_UE1, 3), the method comprising the steps of - establishing a user entity, UE, context for a user entity such that the UE is registered in the MME in which the UE context is established, - appointing another MME in the pool as back-up MME for holding a duplicate UE context of the user entity; - informing (406, 018) the SGW of the back-up MME in which the duplicate UE context is held, or is to be held; and - transmitting (408, 022) a duplicate UE context to the appointed back-up MME, - establishing a user entity, UE, context for a user entity such that the UE is registered in the MME in which the UE context is established, - appointing another MME in the pool as back-up MME for holding a duplicate UE context of the user entity; - informing (406, 018) the SGW of the back-up MME in which the duplicate UE context is held, or is to be held; and - transmitting (408, 022) a duplicate UE context to the appointed back-up MME, characterized in that the step of appointing is performed by - uniquely identifying a particular back-up MME in the pool according to a predetermined function using as input parameters the IMSI of the UE, the number of back-up MMEs in the pool and an order number of the MME in which the given UE context is registered, the function moreover securing that the back-up MME is chosen to differ from the MME in which the UE is registered. - uniquely identifying a particular back-up MME in the pool according to a predetermined function using as input parameters the IMSI of the UE, the number of back-up MMEs in the pool and an order number of the MME in which the given UE context is registered, the function moreover securing that the back-up MME is chosen to differ from the MME in which the UE is registered.