Root Primary or setting switch priority to 0

[MikeNY85]

I am a bit confused between setting a specific switch to be the root using the root-primary command or setting the same switch to have a priority of 0. which command has a stronger effect (root-primary or setting the priority to 0). If a new switch is added to the vlan in both cases, what the effect is going to be if the switch has a lower priority or lower mac-address?.. any help or suggestion will be appreciated.

[Jon Marshall]

Using the primary command is a one off calculation that will set the switch to the lowest priority of all current switches but if you added another switch later with a lower cost as far as I know there is no recalculation so the new switch would become root, probably not what you want. 

If you add a new switch with a lower mac but higher priority nothing happens, but if the switch has a lower priority regardless of the mac address it will become root which is why I set the values manually and make sure any new switches have a higher priority (unless of course I want it to become the root switch). 

Jon

[Joseph W. Doherty]

As I recall (?), root-primary (and root-secondary) just provide Cisco determined low values that usually will cause the switch to be root (or secondary).  However, I also recall, when using different (generations) of Cisco switches, the Cisco values used for root (and secondary) weren't always consistent.  So, to avoid this issue, we (like @Jon Marshall) always used our own (hard coded) root (and secondary) values.

As to using a value of zero, we also decided to not do that, leaving us an "emergency" root value, if we needed to move/reassign the root to a different switch, w/o changing the current root switch's config (or needing it off-line).

[MHM Cisco World · Accepted solution]

there are two method to force one SW to be elect as root 
method 1 primary root
method 2 change priority 

both method can not protect STP domain from elect the new add SW as root when it priority "0" and low MAC. 

so cisco recommend ROOT GUARD config this prevent any new add SW to be elect as root.