Designated Port importance

[lironofir]

On production environment

Why it is important do determine the designated port?

Thanks

[David Ruess]

Hello,

In my opinion I wouldn't just single out Designated ports. I would place importance on learning and understanding all the ports as that's how you determine traffic flow in an STP topology. But to answer your question Designated ports are important because they are all ports from the Root bridge to downstream devices and how traffic from the Root bridge gets to these devices. You also need to know all ports states so when your devices have an STP convergence (which also shouldn't be often or at all) then you know how the traffic will flow once the network has converged.

By definition :

• Designated port—A designated port is a non-root port that is permitted to forward traffic. Designated ports are selected on a per-segment basis, based on the cost of each port on either side of the segment and the total cost calculated by STP for that port to get back to the root bridge. If one end of a segment is a root port, then the other end is a designated port. All ports on the root bridge are designated ports.

Hope that helps

-David

[lironofir]

Thanks,

The downstream can help to rlaborate

[David Ruess]

When most people draw out their topology, especially Access layer STP topology, usually their devices form a triangle. The triangle starts at the top and goes down. So devices connected from the top down are usually referred to as downstream. See attached example.

[MHM Cisco World]

two point 
*1*
some STP feature work only in Designated Port or in SW with Designated port 
*2*
if you see SW other than your topology design ROOT have all port as designated port then this first indication that your topology know have two root / face before elect new root / or the config is wrong.

[lironofir]

Thanks,

can you mention STP feature that works only in designated port?

[MHM Cisco World]

Different between Designated port and  non designated port 

Designated port send bpdu

Non designated port receive bpdu 

So feature like bpdu guard must config in designated port.

NOTE:- BPDU guard need more criteria to apply which is port must connect to Host not to other SW.