Which routing protocol is supported between Cisco ACI spines and IPNs in a Cisco ACI Multi-Pod environment?

OSPF is a widely supported Interior Gateway Protocol (IGP) that can be configured between Cisco ACI spine switches and the Inter-Pod Network (IPN) in a Multi-Pod environment. It is used to advertise the spine switches' TEP addresses, enabling underlying Layer 3 connectivity between pods.

While IS-IS serves as the underlay routing protocol for leaf-spine TEP reachability within an ACI pod, OSPF is a commonly used and explicit choice for direct peering with the external Inter-Pod Network (IPN).

BGP is primarily used for external route distribution from L3Outs and internally as an iBGP route reflector, not as the IGP for underlay reachability between ACI spines and the IPN.

EIGRP, while a supported IGP for the IPN, is a Cisco proprietary protocol, and OSPF is often preferred for its open-standard nature and broader interoperability when connecting ACI spines to a potentially heterogeneous Inter-Pod Network.