IS-IS
Intermediate System to Intermediate System — a link-state routing protocol (IGP) in the same family as OSPF, but with roots in the OSI world. It dominates large operator and data-center networks thanks to its simplicity and scalability.
Like OSPF: it builds a topology database and runs Dijkstra for shortest paths, has a hierarchy (Level 1 = within an area, Level 2 = backbone between areas — corresponds to OSPF's areas). Differences: IS-IS runs directly on L2 (not over IP), which makes it protocol-independent and easy to extend — it was simple to add IPv6 support, one reason operators like it. It's also considered simpler and more robust to scale to very large networks, which is why many Tier-1 operators and hyperscalers run IS-IS as their IGP while enterprises more often run OSPF. Both are valid choices; the choice is often about environment and tradition. Related to OSPF area and administrative distance.