The invention protects the security of a communication between a mobile radio and a radio access network (RAN). A connection is established through the RAN to support a communication with the mobile radio. The connection is configured with a first security configuration. One or more messages are sent over the connection using the first security configuration, each message having a message sequence number. When the connection needs to be configured to a second security configuration, an activation message sequence number associated with the reconfiguration is set. When the reconfiguration process is complete and the second security configuration is to be activated, the next message is sent over the connection with the activation message sequence number. Until that time and during the reconfiguration, when the mobile radio transmits a message with a message sequence number lower than the activation message sequence number to the RAN, it uses the first security configuration. An example of such a message is a cell update message or an area update message.