The STP (Spanning Tree Protocol) port role adopted by a switch port when there is no other port with a lower cost to the root bridge is the Designated Port. when there is no other port with a lower cost to the Root Bridge on a network segment, the switch port on that segment will be designated as the Designated Port for that segment. This ensures that traffic flows efficiently toward the Root Bridge while preventing loops in the network.
Here's a step-by-step explanation:
1. Spanning Tree Protocol (STP):
- STP is a network protocol that prevents loops in Ethernet networks by selecting a single, optimal path for data traffic while blocking all other redundant paths.
2. Root Bridge:
- In a spanning tree network, one switch is elected as the Root Bridge. It is the central switch in the network, and all other switches aim to have the shortest path to the Root Bridge.
3. Port Roles:
- In STP, each switch port assumes one of several roles, including Root Port, Designated Port, and Blocking Port, among others.
4. Designated Port:
- The Designated Port is a switch port on a network segment that has been selected as the preferred port for forwarding traffic to the Root Bridge. It is chosen based on the lowest path cost to the Root Bridge.
5. Path Cost:
- STP calculates the cost of each path from a switch to the Root Bridge based on the bandwidth of the links. The lower the cost, the more preferable the path.
6. Lower Cost to Root Bridge:
- When a switch determines that it has no other port with a lower path cost to the Root Bridge, it will assume the role of a Designated Port for that network segment.
7. Blocking Port:
- Ports that are not designated or root ports and are not part of the active path to the Root Bridge are typically in a Blocking state. They prevent loops by blocking traffic on those ports.
8. Multiple Segments:
- In a network with multiple segments or branches, each segment will have its own Designated Port, ensuring that traffic is forwarded along the most efficient path to the Root Bridge within that segment.