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New Member

How secondary root is identified in the BPDU


When a switch is set as secondary root , the bridge id will come less than root bridge id. How this is notified in the BDPU ? Will it not create a confusion in the root bridge selection. how this is handled in STP ?

VIP Super Bronze

Hi,In the STP root selection


In the STP root selection process, less is better. If Switch A advertises a root ID that is a lower number than the root ID that Switch B advertises, the information from Switch A is better. Switch B stops the advertisement of its root ID, and accepts the root ID of Switch A.

All the switches exchange information for use in the root switch selection and for subsequent configuration of the network. Bridge protocol data units (BPDUs) carry this information. Each switch compares the parameters in the BPDU that the switch sends to a neighbor with the parameters in the BPDU that the switch receives from the neighbor.

have a look at this link for more info:




New Member

Hi Arun,every switch consider

Hi Arun,

every switch consider itself as root till the time it doesnt get any bpdu from its neighbour with lower bridge ID.Once it knows the switch with lowest bridge id, it put that bridge id in its own bpdu's root id field and start communicating to others.

When you configure root primary/Secondary, it automatically decrease/adjust the priority value to make the switch root primary/secondary.However,if you manually assign lowest root value to some other switch then "root primary/Secondary" will not self readjust its value which means your root will no longer act as root.

Hope this will prove helpful.

have fun  :)

Manoj Tyagi

Super Bronze

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There's no "reserved" root or secondary root values.  Root would be the active bridge with the lowest bridge ID and "secondary root" would be the active bridge with the next lowest bridge ID.  As bridge ID also uses device MAC, there shouldn't be any "ties".


As bridge ID also contains a configurable bridge priority, you normally assign a value one or two tiers lower, (or the lowest possible) than the default for your choice of the root bridge and assign a priority between the default and what was set for your choice of root bridge for your secondary root bridge choice.

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