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The purpose of Root Bridge

Posted on 2016-10-29
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Last Modified: 2016-10-29
Knowing that STP is to prevent switching loops.
Well, after you connect the switches together, the STP goes into its calculation to elect the Root Bridge and block redundant  links.  Now will the traffic to the next switch never go from blocked to non-blocked port and vice-versa ?
example:
on the screenshot below, if a PC plugged to Switch B wants to reach a PC plugged to Switch D, will the traffic go B to C then to D or straight B to D.
Also the other way around if PC plugged to Switch D wants to reach PC plugged  to Switch B, will the traffic go D to C then to B or straight D to B?

Thank you
stp
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Question by:jskfan
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Expert Comment

by:Predrag Jovic
ID: 41865144
STP root bridge starts with every switch believes that it is the root bridge. Than - negotiate with other switches who have the the best BridgeID. Winner (Highlander - There can be only one) becomes root bridge (all paths lead to Root bridge), for the rest of the switches rules are relatively simple. Simplified version - Find the best (lowest cost path) to the root bridge (Root port) - block the rest of the paths to the root bridge (not exactly, since there can be downstream switches that path should not be blocked - that's why it is simplified version :) ). In reality similar process is happening between non root bridge switches in the case that there are 2 or more paths to reach Root bridge and both switches have the same cost to reach Root bridge. If cost to reach root bridge is the same again switches compare BridgeID values, whoever looses, needs to block alternative path to the root bridge, however the first criteria is always cost to reach root bridge...
Purpose of all of this is to prevent bridging loops (or switching loops).

You can read more details in article Spanning Tree Protocol: Bridge ID, Priority, System ID Extension & Root Bridge Election Process.
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Author Comment

by:jskfan
ID: 41865151
I understand how root bridge election happens, What I need to understand, is how network traffic is handled as I mentioned in the example:

on the screenshot above, if a PC plugged to Switch B wants to reach a PC plugged to Switch D, will the traffic go B to C then to D or straight B to D.
Also the other way around if PC plugged to Switch D wants to reach PC plugged  to Switch B, will the traffic go D to C then to B or straight D to B?
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Expert Comment

by:Predrag Jovic
ID: 41865163
Traffic will go through unblocked links. In your drawing - without red dots.
From switch B to D path will lead through C. The same is for opposite direction.
(Path B -> D (or D -> B) will not be used as long as path B - D is blocked).
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Author Comment

by:jskfan
ID: 41865175
I have not tested yet ..so if you ping  PC on Switch B from PC on Switch D, ping packets will go through C ?
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Predrag Jovic earned 500 total points
ID: 41865178
Yes in topology with blocked ports marked as above. Block ports can't be used.
Check MAC addresses on ports after ping and you will see the path..
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Author Closing Comment

by:jskfan
ID: 41865228
Thank you
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by:Predrag Jovic
ID: 41865229
You're welcome.
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