Designing network update algorithms is an important line of research in developing software for new-generation telecommunication networks—software defined networks (SDNs). A specific case of the network update problem is seamless recovery of network configurations after the loss of some forwarding rules, as can happen, e.g., due to expiry of their timeouts. This paper initiates a systematic study of this problem using a formal SDN model. As a result, two algorithms a presented for safe and correct insertion of lost forwarding rules in SDN flow tables and it is shown that in the general case, seamless recovery of network configurations cannot be achieved without forwarding rules with multiple priorities.