Refer to the exhibit.

In a Cisco ACI fabric, Spanning Tree Protocol (STP) Bridge Protocol Data Units (BPDUs) that are generated by an external switch, such as Switch2 in this scenario, are indeed forwarded across the fabric. This allows externally connected switches to maintain a loop-free topology. While Cisco ACI does not participate in STP as it uses its own protocols and mechanisms for loop prevention within the fabric, it will forward STP BPDUs across End Point Groups (EPGs) on which they are received1. This ensures that the connected external switches can still use STP for their loop prevention mechanisms.

References :=

Cisco Community Discussion on STP BPDUs in ACI1