Interactions outside of the MSTP region

A boundary port on an MST switch is a port that receives an STP (version 0) BPDU, an RSTP (version 2) BPDU, or a BPDU from a different MST region.

If the port receives a version 0 BPDU, it will only send version 0 BPDUs on that port. Otherwise, it will send version 3 (MST) BPDUs because the RSTP switch will read this as an RSTP BPDU.