OSPF (Open Shortest Path First) is the most popular among the link-state routing protocols. The current IPv4 version, OSPFv2, is widely deployed throughout carrier, ISP, and enterprise networks. OSPFv3 essentially is an IPv6-enabled OSPF. It is a well documented protocol in terms of standards, books, guides, and white paper density.

The knowledge level is high among those who deploy and operate OSPFv2. Integrated IS-IS, however, is catching up in popularity, although those familiar with its addressing already appreciate its simplicity. OSPF is multicast-based ( = AllSPFRouters, = AllDRouters).

OSPF facilitates 11 different LSA types. This is the biggest obstacle and source for confusion that OSPF apprentices face. OSPF and IS-IS both support Equal-Cost Multi-Path (ECMP), an important feature for optimal link utilization. OSPF uses an arbitrary metric (cost) that is based on link bandwidth in the Cisco implementation.

The Cisco IOS implementation will do equal-cost load balancing for up to six equal-cost routes for OSPF and IS-IS. This condition is characterized by including all routes in the forwarding table. Route tagging and authentication are additional features of modern implementations. Traffic-engineering extensions to OSPF (OSPF-TE) via TE-LSAs are available as well. Traffic engineering in combination with leaf-area concepts is particularly difficult to implement, given the intrinsic behavior of link-state routing protocols.