Which Layer 3 protocol is used within the ACI fabric?

OpenFlow is an SDN protocol used for controlling network devices, but it is not the Layer 3 routing protocol used within the ACI fabric's underlay.

EIGRP (Enhanced Interior Gateway Routing Protocol) is a Cisco proprietary routing protocol, not the chosen underlay routing protocol for the ACI fabric.

OSPF (Open Shortest Path First) is a common interior gateway protocol, but it is not the routing protocol used internally within the ACI fabric's underlay, which specifically uses IS-IS.

Cisco ACI uses an IS-IS (Intermediate System to Intermediate System) based routing protocol within the fabric as its underlay Layer 3 protocol. This allows the APIC to centrally manage and configure the fabric's routing table without manual configuration of individual leaf and spine nodes, providing a scalable and robust routing foundation.