STP Designated Ports & Root Ports

Answered Question
Dec 27th, 2007

All,

This an easy STP question for someone but am I summarizing the operation of root ports and designated ports correctly when I say root ports carry bpdu traffic away from the root bridge while designated ports bring bpdu traffic to the root bridge. If this summarization is incorrect please correct my thinking. Thanks.

I have this problem too.
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Correct Answer by Edison Ortiz about 9 years 1 month ago

You aren't being funny and that's a very valid question.

The answer is it depends. If the non-root switch has ports connected to other non-root switches and those non-root switches do not have a direct connection to the root switch, they will traverse via another non-root switch.

For instance

[root]->[non-root]<->[non-root]

On this scenario, the middle non-root switch will have traffic via 2 ports. On this switch, one port will be set to root (pointing to the root switch) while the other port will be set to designated (pointing to the other non-root switch).

On this other non-root switch, it will have a port designated as the root port (connected to the non-root switch) but on STP it will have the information of the real root switch which is 2 hops away.

HTH,

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Edison Ortiz Thu, 12/27/2007 - 08:24

Hmm..

A root port can only be located in a non-root switch. On this switch, that's the port connected to the root switch.

On this non-root switch, you will also find other designated ports connected to end-devices (workstations, printers, etc).

On the root-switch, there won't be any root port. All ports are designated ports.

As for BDPU, any inter-switch link will carry BPDU as long as STP is enabled on both switches. Understanding this process as "carrying away" and "bring traffic to" can be quite confusing and does not depict how it really works.

Just understand the function of each device and everything will make sense.

mrashby Thu, 12/27/2007 - 08:45

Okay, with that said is there only one root port per non-root switch and if so does bpdu traffic only or all traffic flow across the root ports on the non-root bridges? Not trying to be funny just trying to understand how it works.

Thanks.

Correct Answer
Edison Ortiz Thu, 12/27/2007 - 09:12

You aren't being funny and that's a very valid question.

The answer is it depends. If the non-root switch has ports connected to other non-root switches and those non-root switches do not have a direct connection to the root switch, they will traverse via another non-root switch.

For instance

[root]->[non-root]<->[non-root]

On this scenario, the middle non-root switch will have traffic via 2 ports. On this switch, one port will be set to root (pointing to the root switch) while the other port will be set to designated (pointing to the other non-root switch).

On this other non-root switch, it will have a port designated as the root port (connected to the non-root switch) but on STP it will have the information of the real root switch which is 2 hops away.

HTH,

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