In this paper, we analyze the effect of phase noise on the downlink ergodic sum-rate of a cloud radio access network. The system comprises of one baseband processing unit (BBU) on the cloud server which coordinates M multi-antenna remote radio heads (RRHs) serving K single-antenna users using regularized zero-forcing precoding. We assume the BBU has all users' data and imperfect channel state information and communicate with RRHs via optical fibers which are referred to as fronthaul links. The effect of phase noise both at RRHs and users is also taken into consideration. A deterministic approximation of downlink ergodic sum-rate is derived based on large dimensional random matrix theory when the numbers of antennas at RRHs and users are asymptotically large with a fixed ratio. From simulation results, it is confirmed that the deterministic approximation is accurate and the effect of phase noise is shown to result in a significant reduction in system performance.