Mobile ad hoc networks are infrastructure-free, pervasive and ubiquitous in nature, without any centralized authority. These characteristics tied with the growing concerns for security attacks. This paper provides simulation experiments that describe various routing misbehavior that are specific to MANET and the effects of these routing attacks on the network performance. The paper is focused on the evaluation of network performance in terms of packet delivery fraction, average end-to-end delay and global throughput of a mobile ad hoc network where a defined percentage of nodes behave maliciously. The simulation results show that network operation and network performance severely degrade when no countermeasures are taken.