The minimum MTU value that must be supported by the ISN to allow control plane endpoint information exchange between sites in a Cisco ACI Multi-Site setup with default settings is 1554 bytes (B). This accounts for the standard MTU of 1500 bytes plus additional overhead for VXLAN encapsulation. An MTU of 9000 (A) or 9100 © is not the minimum required, and 1600 (D) is not related to the default fabric settings5. References := (Multi-site / Multi-Pod MTU Jumbo Frames5)

 In a Cisco ACI Multi-Site setup, the ISN (Inter-Site Network) connects different ACI sites. For efficient multicast traffic routing, BIDIR-PIM (Bidirectional Protocol Independent Multicast) is configured throughout the ISN, which is why option B is correct. Increasing the ISN MTU (Maximum Transmission Unit) to support the maximum MTU sent by endpoints is crucial to prevent packet fragmentation or loss, making option C correct.

Options A, D, and E are not typically required for an ACI Multi-Site setup. Direct connectivity between Spine switches and ISN routers, as mentioned in options A and D, is usually established through leaf switches. Option E, enabling IGMPv3 on ISN routers, is more relevant for managing multicast subscriptions within a single site.

References: = Implementing Cisco Application Centric Infrastructure Official Cert Guide

When traffic to certain endpoints is dropped in a Cisco ACI fabric configured with hardware proxy, setting the L2 Unknown Unicast option to Flood for Layer 2 switched traffic can prevent this problem. This mode allows unknown unicast traffic to be flooded within the bridge domain, rather than being sent to the spine proxy and potentially discarded if the destination is unknown5.

References := Solved: Unknown Unicast in ACI Fabric - Cisco Community