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Name:JoshuaJoin In:Mar 6, 2019
[SOLVED] Other STP knows, but I don’t know the election process of “designated port”

Post by Joshua » Mar 2, 2019


STP is a difficult point in the NA phase. I am also learning over and over again, hoping to understand how it works.

I knew the first two steps of the STP election, but I can't understand the last step: how the election "designated port" works.

The first step is to elect the "root bridge," a term that is easy to understand, that is, the switch at the root of the tree topology.

The second step is to elect the "root port", which is also easy to understand, that is, the path closest to the root bridge of each non-root switch.

And in the third step, the election of the "designated port", it is confused, after selecting the designated port, what should be done to "designate" the port?

As shown below:

For this topology, the first step is to elect the root bridge. Because SW1 has the smallest BID, it becomes the root port.

The second step is to elect the root port. First, the Fa0/1 of SW2 and the Fa0/2 of SW3 are directly connected to the root bridge. If the cost of the two ports is the same, they become the root bridge.

But in this third step, I voted for the designated port. I can't use the knowledge I learned to interpret the next process, so I can get the three designated ports in the above figure.

I have checked a lot of information on the Internet and found that all the data in China are based on three switches, but if I expand to four switches, I can't interpret them.

Ask for advice here!


The root port receives the BPDU and specifies the port to forward the BPDU.

Mar 3, 2019

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