Designated Port - STP

Answered Question
Nov 6th, 2007

Hi,

I'm studing for the BCMSN exam.

I have a doubt about when does a port is assigned the roll of Designated Port. I know the theory about it, but I would like an explanation in somebody's own words to have a better understanding.

regards...

Correct Answer by compsolv about 9 years 3 months ago

I hope this explanation will help you….here is the process that spanning tree uses in order to prevent loops. First, a root bridge is selected based upon the Bridge ID value. Next, one root port is selected on each non-root bridge. This selection is based upon the best bandwidth path back to the root. Next, one designated port is selected on each link. Again, bandwidth is the first value analyzed to make the decision. All remaining ports are considered non-designated and are placed in the blocking state.

I look forward to any follow up questions that you might have!

Anthony J. Sequeira

#15626

Don't Cheat Yourself - LEARN!

http://www.netmasterclass.net/CCIE/Self-Paced-Program/Written

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Overall Rating: 3.5 (2 ratings)
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erikjustinlee Tue, 11/06/2007 - 13:34

Designated Port is nothing more than the "prefered" port to get back to the root switch.

Correct Answer
compsolv Tue, 11/06/2007 - 14:43

I hope this explanation will help you….here is the process that spanning tree uses in order to prevent loops. First, a root bridge is selected based upon the Bridge ID value. Next, one root port is selected on each non-root bridge. This selection is based upon the best bandwidth path back to the root. Next, one designated port is selected on each link. Again, bandwidth is the first value analyzed to make the decision. All remaining ports are considered non-designated and are placed in the blocking state.

I look forward to any follow up questions that you might have!

Anthony J. Sequeira

#15626

Don't Cheat Yourself - LEARN!

http://www.netmasterclass.net/CCIE/Self-Paced-Program/Written

ccnp_seeker Wed, 11/07/2007 - 05:07

thanks both for the explanations....I'm sure now I have the right concept :)

swmorris Sat, 11/10/2007 - 08:20

Anthony,

It's not just the best bandwidth path back to the root. That's simply the first criteria used. In case of tie, received bridge ID is used. In case of further tie, port priority/ID is used.

Root and designated ports use the same election process, just the perspective is different. Root port is from the perspective of the switch itself. Designated port is from the perspective of the link (or a device sitting in the middle of a link).

HTH,

Scott

smorris@ipexpert.com

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