Enabling ARP flooding is necessary when VMs are migrated from a pre-existing network to Cisco ACI and their gateways remain in the legacy setup. This allows the ACI fabric to learn about silent hosts that do not initiate communication, ensuring that the migrated VMs can communicate with all hosts in the network, including those that are silent.

The topology change notification (TCN) in Spanning Tree Protocol (STP) triggers a flush of the MAC address table. When a leaf node receives a TCN, it flushes the MAC address entries associated with the VLANs affected by the topology change. In this case, since the TCN is related to VLAN 101, the ACI fabric will flush all endpoints associated with VLAN 101 in bridge domain BD10123 on the leaf node.

The control plane TEP (Tunnel Endpoint) IP address is a unique IP address assigned to each spine node within a Cisco ACI fabric. This IP address is used to establish MP-BGP EVPN (Multiprotocol Border Gateway Protocol Ethernet VPN) and VPNv4 (VPN version 4) adjacencies with the spine nodes in remote sites when configuring Cisco ACI Multi-Site. The control plane TEP IP enables the spine nodes to exchange routing information and network reachability for seamless integration of multiple sites into a single ACI fabric. References := Implementing Cisco Application Centric Infrastructure Official Cert Guide