MANET, due to the nature of wireless transmission, has more security issues compared to wired environments. In this paper, the authors specifically considering tunnelling attack which does not require exploiting any nodes in the network and can interfere with the route establishment process. Wormhole attack is a network layer attack observed in MANET, which completely disrupts the communication channel. Instead of detecting suspicious routes, they suggest a new method which detects the attacker nodes and works without modification of protocol, using a hop-count analysis from the viewpoint of users without any special environment assumptions. The tunnelling attack is simulated using OPNET and proposed work showing the detection and isolation algorithm.