The scenario involves an application (App_1) on server S1 and a silent host application (App_2) on S2, both using the same VLAN encapsulation. The task is to force the ACI fabric to learn App_2 on Leaf-2. A silent host does not generate traffic, so special handling is needed.

Requirement Analysis

A silent host requires flooding (e.g., ARP or unknown unicast) to be learned by the fabric when it moves or is detected.

The goal is to ensure Leaf-2 learns App_2's endpoint.

Option Evaluation

A . Set Multi-Destination Flooding to Drop:

Dropping multi-destination traffic prevents learning, which is counterproductive for a silent host.

B . Set Unicast Routing to Hardware Proxy:

Hardware proxy optimizes unicast routing but does not force learning of a silent host via flooding.

C . Set L2 Unknown Unicast to Flood:

Enabling L2 unknown unicast flooding causes the fabric to flood traffic (e.g., ARP) across the bridge domain, allowing Leaf-2 to learn App_2's MAC address even if it is silent.

D . Set L3 Unknown Multicast to Optimized Flood:

This applies to multicast traffic and is irrelevant for learning a silent host's MAC address.

Final Answer Justification

C is correct because flooding L2 unknown unicast traffic ensures the ACI fabric learns App_2 on Leaf-2 by propagating ARP or other discovery traffic.

Primary Cisco Reference:

Cisco ACI Endpoint Learning Guide, 'Flooding for Silent Hosts.'

Cisco APIC Bridge Domain Configuration Guide.