An engineer must design Layer 2 connectivity for a subnet between two data centers. The engineer must ensure seamless host mobility and optimal first-hop routing. Which attribute must be synchronized

While synchronizing NX-OS versions is good practice for operational consistency, it does not directly enable seamless host mobility or optimal first-hop routing in an extended Layer 2 subnet.

DCNM (Data Center Network Manager) is a management platform for network fabrics, not an attribute that directly facilitates host mobility or optimal routing in the data plane.

Synchronizing a multicast group might be relevant for specific multicast applications in a stretched Layer 2 network, but it is not the primary attribute that ensures seamless host mobility and optimal first-hop routing for all hosts.

Synchronizing the anycast gateway MAC address across all first-hop routers in different data centers for the same Layer 2 extended subnet is crucial for seamless host mobility. This allows hosts to move between sites without needing to update their ARP cache, always sending traffic to the same MAC address which is then handled by the local (closest) active gateway for optimal first-hop routing.