What is required for EIGRP neighbors to communicate successfully?

For Enhanced Interior Gateway Routing Protocol (EIGRP) neighbors to communicate successfully, they must have matching Autonomous System (AS) numbers. The AS number is a crucial identifier that groups routers into the same administrative domain for routing purposes. When routers exchange routing information, they will only establish neighbor relationships with routers that share the same AS number, as this ensures the information they exchange pertains to the same routing domain and policies.

In the context of the other choices, while interface types, metrics, and subnet masks play roles in routing and neighbor relationships, they do not solely determine whether EIGRP neighbors can communicate. For example, routers may have different types of interfaces and still successfully form neighbor relationships as long as their AS numbers match. Similarly, although metrics are important for route selection, they do not affect the establishment of neighbor relationships, nor does having identical subnet masks. Thus, a matching AS number is the fundamental requirement for EIGRP neighbor communication.