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The locator/ID separation protocol (LISP) proposed for addressing the scalability issue of the current Internet has gained much interest. LISP separates the identifier and locator roles of IP addresses by end point identifiers (EIDs) and locators, respectively. In particular, while EIDs are used in the application and transport layers for identifying nodes, locators are used in the network layer for locating nodes in the network topology. In LISP, packets are tunneled from ingress tunnel routers (ITRs) to egress tunnel routers in a map-and-encapsulation manner. For this purpose, an ITR caches on demand some mappings between EIDs and locators. Since hosts roam from place to place, however, their EID-to-locator mappings change accordingly. Thus, an ITR cannot store a mapping permanently but maintains for every mapping a timer whose default value is set to a given cache timeout. If the cache timeout for a mapping is too short, an ITR frequently queries the mapping system (control plane), resulting in a high traffic load on the control plane. On the other hand, if the cache timeout for a mapping is too long, the mapping could be outdated, resulting in packet loss and associated overheads. Therefore, it is desirable to set appropriate cache timeout for mapping items. In this paper, we analytically determine the optimal cache timeout for EID-to-locator mappings cached at ITRs to minimize the control plane load while remaining efficient for mobility. The results presented here provide valuable insights and guidelines for deploying LISP.