In this paper, we investigate the effect of a LAN traffic on the performance of a shared buffer ATM switch andIWF (interworking function )on a LAN environment through simulations. Firstly, the delay and the buffer occupancy of the switch and IWF are mesured according to the proportion of the LAN traffic to the traffic generated by gernoulli process. Secondly, we investigate the behavior of the switch in the case that LAN traffic is concentrated to a connectionless server, and the effect of LAN traffic shaping at IWF on the delay and the buffer occupancy of the switch.