What is OSPF!! | IPCisco.com . OSPF (Open Shortest Path First), is one of the most used Routing Protocol today. OSPF is a standard based Link-State routing protocol. It is a Classless routing protocol which do not send subnet masks with updates. So, it allows to use VLSM. Open Shortest Path First has similarities with the other Link-State routing protocol IS-IS but it differents from it in many points. Open Shortest Path First uses a special algorithm, Dijsktra Alogirthm for route calculations.