How is the designated port role determined on a boundary interface?

The designated port role on a boundary interface is determined by comparing Bridge Protocol Data Units (BPDUs) from Multiple Spanning Tree (MST) and Per VLAN Spanning Tree (PVST+). When a boundary switch interfaces between these two spanning tree protocols, it needs to make a decision on which port will serve as the designated port for forwarding traffic.

This process involves analyzing the BPDUs sent from both MST and PVST+ instances. The switch compares the information contained in these BPDUs, which includes details like the root bridge ID and port costs, to determine which protocol will have the designated port. This comparison is crucial for maintaining stable and efficient network operation, ensuring that loops are prevented while allowing for optimal data flow.

By utilizing the comparison method with BPDUs, the switch can effectively manage its ports according to the network topology and the attributes of the competing spanning tree protocols, leading to a more reliable and organized forwarding network.