STP Primary or Priority

jeff.cook · ‎03-17-2009

When configuring STP is it better to use the "Primary" option or to statically set the priority? The Cisco docs say to use primary and secondary, but I have always heard to set everything as static as possible.

Just looking for your thoughts.

Thank You

Jon Marshall · ‎03-17-2009

Jeff

Different people will probably recommend different things.

If you use the primary option then the switch will calculate the value at the time you enter the command and ensure that it chooses a lower value than any other. Note that if a switch was then introduced with a lower value than this then the original switch would not recalculate.

Personally i have always used static values ie. 8192 for primary and 16384 for secondary. Has always worked well for me.

Jon

Francois Tallet · ‎03-19-2009

I would also recommend the use of the priority.

First, there is no command to make sure that you are the root bridge.

Furthermore, the root command is in fact a macro that is expanded in several command. The main use of this macro is not to influence the root bridge election, but rather to tune the timers depending on the diameter entered as a parameter (along with a hello-time). This STP timer tuning is not really relevant any more with RSTP and MST.

You don't need to tune the timer, so just set the priority and you're doing exactly what the root macro does;-)

Regards,

Francois

bs6825 · ‎03-19-2009

I agree with the prior posts about setting the priority statically but would like to add one more tidbit. Before setting the priority to any value 8196, 4092, etc, be sure to run a "show spanning-tree" command. Verify the priority of the current root bridge and make sure the value you set is LOWER to ensure that switch becomes the root. Remember this value for setting your secondary with a value betweent he old root and the new one.

Good Luck