MST (IEEE 802.1s), BPDU frames do not carry a VLAN tag, and the BPDUs are sent over the native VLAN. Typically, the native VLAN is not used to carry data traffic, and the native VLAN may not be configured for data traffic on the Cisco ACI fabric. As a result, to help ensure that MST BPDUs are flooded to the desired ports, the user must create an EPG (an MST EPG) for VLAN 1 as native VLAN to carry the BPDUs. This EPG connects to the external switches that run MST.
In addition, the administrator must configure the mapping of MST instances to VLANs to define which MAC address table must be flushed when a Topology Change Notification (TCN) occurs.
When a TCN event occurs on the external Layer 2 network, this TCN reaches the leafs to which it connects via the MST EPG, and flushes the local endpoint information associated with these VLANs on these leafs; as result, these entries are removed from the spine-proxy mapping database.
https://www.cisco.com/c/en/us/solutions/collateral/data-center-virtualization/application-centric- infrastructure/white-paper-c11-737909.html