Why do all my gigabit and 10G links show the same OSPF cost?

OSPF cost = reference bandwidth ÷ interface bandwidth, and the default reference is 100 Mbps. FastEthernet: 100/100 = 1. Gigabit: 100/1000 = 0.1 → rounded up to 1. 10G: also 1. Everything at or above 100 Mbps costs the same, so modern links look identical and OSPF happily ECMPs across paths you consider unequal. Fix: raise the reference on every router in the domain — auto-cost reference-bandwidth 100000 (i.e., 100G) makes 10G=10, 1G=100, and path selection meaningful again. Consistency matters: routers with different reference bandwidths compute incomparable costs and you get asymmetric or looping-ish path choices. Per-interface ip ospf cost N remains the surgical override for traffic engineering a single link.