This paper presents an analytical model for quantifying the reactive routing overhead in Mobile Ad-hoc Networks (MANETs). The overhead is characterized as a function of network mobility and traffic load. Also, a numerical model for determining the optimal cached route Time-To-Live is derived to minimize the overhead depending on network mobility and traffic load. The final result shows the minimum communication bandwidth in bits necessary to facilitate reactive routing. This paper provides a valuable insight into the fundamental limits of routing performance in MANETs in response to mobility.