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Maintaining service availability during an inter-domain route update is a challenge in both conventional networks and software-defined networks (SDNs). In the update process, asynchronous reconfigurations to border forwarding devices in different domains will incur transient anomalies with numerous packet losses and service disruptions. Based on current SDN inter-domain routing mechanisms, we in this paper propose a lossless and seamless method for SDN inter-domain route updates. This method is lightweight, and it has no requirement to add extra switch functionality or to extend SDN southbound protocols. The primary idea of this method is to achieve a lossless inter-domain route update by communications and collaborations among relevant domains. Motivated by this idea, we first identify three different domain categories for the update, i.e., domains only on the new inter-domain route, domains on both the old and new inter-domain routes, and domains only on the old inter-domain route. We further find that the transient anomalies are able to be avoided by reconfiguring the related border switches of the three categories of domains in order. Four update steps are then designed to keep the orderly update. Furthermore, we present the theoretical proof of the effectiveness of this method. Finally, based on our prototype implementation, the proposed method is also validated by simulation studies, and the simulation results indicate that this method succeeds in avoiding packet loss and maintaining service availability during the update. [ABSTRACT FROM AUTHOR]