The technology disclosed herein enables the automatic discovery of MTU size across a software defined network (SDN). In a particular embodiment, a method provides, in a management plane of the SDN, receiving a first MTU request from a first endpoint of the SDN that indicates a second endpoint of the SDN to which the first endpoint will transfer first data. The method further provides tracing a first path through the SDN for the first data between the first endpoint and the second endpoint, determining the smallest MTU along the first path, and setting a first MTU to a value less than or equal to the smallest MTU along the first path. The method also includes providing the first MTU to the first endpoint.