Well, the bad news is that not all routing protocols support VLSM. Classful routing protocols such as RIPv1 and IGRP, do not support VLSM. Therefore, it’s important to ensure that you configure your router for VLSM with one of the supported protocols. But the good news is that all current generation of routing protocols such as RIPv2/v3, OSPF, IS-IS, EIGRP,  BGP, and even Static routes, are classless and therefore support VLSM.