Scalability is a key issue of IoT, as the number of smart devices increases day by day due to smart city development. The standardized IP-based routing protocol used in the IoT is the RPL. However, it is affected by scalability issues. There is no possibility to store routes to root nodes in the routing tables over large-scale networks. It exacerbates the problem of energy depletion, especially in the nodes closer to the root, because it has more descendants than its children. There are diversified works developed to enhance the RPL in the aspect of network scalability. They combine multiple factors in OFs design and disable the storage of all subtree nodes in the DODAG graph. However, it still needs to develop an efficient and effective scalable RPL protocol for IoT scenarios. The scalable RPL routing protocol has to support the increasing connected IoT device environment, the number of users, modernized application features, and different analytic capabilities without compromising the actual performance.