What must be present for each collision domain in the Spanning Tree Protocol (STP)?

In Spanning Tree Protocol (STP), each collision domain must have one designated port. The designated port is crucial because it is responsible for forwarding frames onto the segment where it resides. This port is selected based on which bridge has the lowest path cost to the root bridge, ensuring that the data is sent efficiently through the network.

The designated port serves to eliminate the potential for network loops, which can occur in environments where there are multiple active paths to the same network segment. By ensuring there is only one designated port per collision domain, STP effectively manages the topology of the network, allowing for efficient data flow while preventing broadcast storms and other issues associated with loops.

In contrast, while it's important for STP to manage ports like root ports and non-designated ports, the presence of the designated port is what specifically addresses the forwarding of traffic within each collision domain, making it essential for the operational integrity of the network.