An engineer configures a Multi-Pod system with the default getaway residing outside of the ACI fabric for a bridge domain. Which setting should be configured to support this requirement?

When a Bridge Domain (BD) is configured with its default gateway residing outside the Cisco ACI fabric, the fabric acts as a Layer 2 transit network for that specific subnet. To support this Layer 2-only deployment model, Unicast Routing must be disabled on the BD. Disabling Unicast Routing prevents the ACI leaf switches from acting as the default gateway, stops the fabric from learning IP addresses for routing purposes within that BD, and ensures that all routing functions and ARP resolutions are deferred to the external gateway device.

A. disable Limit IP Learning to Subnet: This setting restricts IP learning to configured subnets; it does not address the fundamental requirement of disabling routing for an external gateway.
B. disable IP Data-plane Learning: While this stops data-plane IP learning, disabling Unicast Routing is the comprehensive and required setting for a purely Layer 2 BD.
D. disable Advertise Host Routes: This prevents host route advertisement to external peers, which is irrelevant if the BD is not performing routing in the first place.

1. Cisco APIC Layer 2 Networking Configuration Guide, "Bridge Domain Configuration" section.

2. Cisco Application Centric Infrastructure Design Guide, "Layer 2 Forwarding and Bridge Domains".