Designated ports
There can be only one Root Port (marked as RP) on a Switch, but a Switch can have multiple Designated ports (marked as DP). The Designated Port is the port that has the lowest Path Cost on a particular Local Area Network (LAN) segment. Each segment has a single port that is used to reach the Root Bridge (Root Switch) called Designated Port. A Root Port can never be a Designated port.
A Root Port is the port on the Switch with the least cost from the “Switch” to the Root Bridge. A Designated Port is the port on a “Local Area Network (LAN) segment” with the least cost to the root bridge. The other end of a Designated Port is called as Non Designated Port (marked as NDP), if it is NOT a Root Port. Non Designated Port will be always in Blocking State, to avoid Layer 2 Switching loops.
Remember, a Root Port can never be a Designated Port and also there cannot be any Root Port on a Root Bridge (Root Switch). All the ports on a Root Bridge (Root Switch) are Designated Ports.
Root ports
Once the Root Bridge (Switch) is elected, every other Switch in the network must select a single port on it to reach the Root Bridge (Switch). The single selected port on a Switch with least Path Cost to the Root Bridge is called the Root Port. Root Bridge (Switch) will never have a Root Port. Root Bridge (Switch) is at the Root and therefore there is no need of a Root Port to reach the root.
The above layout of Switches shows that Switch 4 has two ports to reach the Root Bridge. If there are multiple ports present in a Switch to reach the Root Bridge (Switch). Spanning Tree Protocol Algorithm must select the best port from them to reach the Root Bridge. Here the port with least path cost (4+4=8) is marked as Root Port.
Different between Root port and designated port
Following figure shows a network with four Switches.The differences between Root Port and Designated Port are listed below.
• Root Port is a single selected port on a Switch, other than Root Switch, with least Path Cost to reach the Root Bridge. The Designated Port is the port that has the lowest Spanning Tree Path Cost on a particular Local Area Network (LAN) segment.
The Root Port is the port on the Bridge (Switch) with the least Spanning Tree Path Cost from the switch to the Root Bridge. A Designated Port is the port on a Local Area Network (LAN) segment with the least Spanning Tree Path Cost to the Root Bridge (Root Switch).
• There can be ONLY one Root Port on a Bridge (Switch). There may be multiple Designated Ports on a Bridge (Switch).
All the ports on a Root Bridge (Root Switch) are Designated Port and there is no Root Port on a Root Bridge (Root Switch).
• A Root Port can NEVER be a Designated Port.
• If one end of a Local Area Network (LAN) segment is a Designated Port, other end is called as Non Designated Port (marked as NDP), if it is NOT a Root Port. Non Designated Port will be always in Blocking State, to avoid Layer 2 Switching loops.