Vehicular Ad-hoc Network (VANET) is a rising & most challenging research area to provide Intelligent Transportation System (ITS) services to the end users. The implementation of routing protocols in VANET is an exigent task as of its high mobility & frequent link disruption topology. VANET is basically used to provide various infotainment services to each and every end user; these services are further responsible to provide an efficient driving environment. At present, to provide efficient communication in vehicular networks several routing protocols have been designed, but the networks are vulnerable to several threats in the presence of malicious nodes. Today, security is the major concern for various VANET applications where a wrong message may directly or indirectly affect the human lives. In this paper, we investigate the several security issues on network layer in VANET. In this, we also examine routing attacks such as Sybil & Illusion attacks, as well as available solutions for such attacks in existing VANET protocols.