The switch with the best path to the root switch is set to forwarding. That switch is referred to as the designated switch, and its port is referred to as the designated port. A designated port is a port that can have the lowest path cost on a Local Area Network(LAN) segment. Each segment has a port called a single port that is used to reach the root switch or root bridge.
A bridge device is equipped with two (or more) ports. The one connected on side where the STP root is located is referred to as the ‘root port.’ A ‘designated port’ is a port that does not face the root but forwards traffic from another segment at the lowest possible cost.
Working:
- The root bridge is Switch 1.
- The root port (RP) is the port that connects to the root bridge, therefore there are no root ports on the root bridge. Every port on a root bridge forwards, and they are all designated ports (DP).
- When the switches power up, they exchange BPDUs and select a root bridge as the first thing they do. In this scenario, Switch 1 is chosen.
- Following that, each switch must determine shortest way to root bridge. I haven’t added expenses for each link in this diagram, however, based on the information above,
- Switch 3 considers its direct connection to Switch1 to be least expensive, therefore port on that link becomes the RP.
- Switch 2 considers its direct connection to Switch 1 to be the least expensive, therefore port on that link becomes RP.
- After the switches have agreed on root bridge and respective RPs, they must now locate their assigned ports. The selected ports are in charge of forwarding traffic onto a network segment. Consider following:
- The RPs lead to the root bridge.
- DPs are paths that lead away from the root bridge.
- Because the root bridge does not have any blocking ports, all of its ports are DPs.
So the only other section not included in the figure is Switch 2 -> Switch 3. One of the ports connecting that segment must forward traffic to that segment; otherwise, that segment will never receive traffic. But they can’t both forward since it would create a loop, i.e. a packet sent from Switch 1 would travel to Switch 3 -> Switch 2 and back to Switch 1 and so on.
So Switch 2 and Switch 3 compare prices on that segment in the BPDUs delivered between them, and one of them, in this example Switch 3, has a lower cost. It forwards its port, transforming it into a DP. Switch 2 must now put its port into blocking mode to break the loop.
Some major key points:
- The switch with the lowest cost to the root is selected as the link’s designated switch.
- A switch can have multiple designated ports.
- All ports on a Root Bridge (Root Switch) are designated ports.
- If one end of a LAN segment is a Designated Port, the other end is referred to as a Non-Designated Port (marked as NDP) if it is not a Root Port.
- A root port can never be a designated port.
- A bridge can only have one Root Port (Switch). A bridge may have numerous Designated Ports (Switch).