STP question

Unanswered Question
Feb 13th, 2008


what is use of spanning tree root primary and spanning tree root secondary command. if any new switch with insert into insert into network with prioirty less than secondary what would happend in that case

I have this problem too.
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hi vinod,

'spanning tree root primary' actually force your

bridge/switch to become primary root bridge. As son as you input this command the priority of the bridge becomes one less than the priority of the existing root bridge. Thus it becomes the new root of the STP topology.

'spanning tree root secondary' forces the switch to become primary as soon as the existing primary root switch steps down / goes-down.

if any new switch which insert into network with prioirty less than secondary ==> it should cause the root secondary's priority to become less than the new switch, but I'm not sure. Any ideas gentlemen.........!


shrikar.dange Thu, 02/14/2008 - 00:39

hi vinod,

with spanning-tree root primary and secondary commands you can configure these particular switch to be tha root and backup root bridges respectively without manually configuring the priorities.

When you configure the root to be primary then the priority of this switch becomes lesser than the current root bridge.(If the current root's priority is less than 24576 then the primary roots priority becomes 4096.)By defualt the priority will be set to 24576.For secondary root the priority will be any value less than 28762.

If now another switch comes to the picture whose priority is less than secondory but more than primary nothing happens until primary fails.But if the primary root fails then secondary root will not become the root this new switch will take over as a root.(Because there is no way you can find what priority the secondary bridge is using and switches can not change the prority dynamicly once STP is fully converged.)

Now if the priority of the new switch is less than the primary root or less than 4092 then new switch becomes the root!!!!!(Unless u have applied root guard,bpdugurad etc features).



shri :)

Francois Tallet Thu, 02/14/2008 - 09:43

Selecting the root bridge by setting the bridge priority is straightforward, once you have understood that the lower value the better.

That's why I think the real value of the spanning-tree root macro is not really in how it helps selecting the root bridge. It's rather how it helps tuning the STP timers (a much more complex problem than selecting the root with the priority command;-)

The spanning-tree root CLI is in fact a macro, which means that it expands into several IOS commands. Not only it will set the bridge priority but it will also tune the max-age and forward-delay according to the diameter specified (if no diameter is given as an argument to the macro, a default value is assumed).

I don't recommend you use this command with RSTP or MST as tuning aggressive values for max-age or forward-delay is seldom useful in that case.




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