The resource-constrained nature of IoT is vulnerable to various types of RPL routing attacks. Hence, securing the RPL routing is essential to accomplish better network performance. Recent research on RPL security is increased due to its ease in deploying in several critical applications such as E-health, smart homes, and smart cities. IoT Things used in smart cities and homes are intractable with the Internet. These things are deployed in a distributed environment, and they are connected wirelessly. They are vulnerable to diverse security attacks, especially in the routing layer. The basic RPL has been defined with no security features. The stated RPL security problem severity becomes higher in smart cities and E-health applications. Moreover, IoT routing security is a challenging task over tiny sensor devices.