spanning tree question

Unanswered Question
Sep 23rd, 2010

hi all, If I have one link to say an edge a switch which is currently the root switch, If I plug another switch in the root with a lower priority. will the port on the edge switch go into blocking mode once it hears the new bpdu ? if so why is this even if there is only one link to the switch ?


I have this problem too.
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Alan Taylor Thu, 09/23/2010 - 15:36

Heya Carl,

No I think the new switch with the lower priority will try to advertise but will be "trumped" by the current root switch and then transition the port to forwarding and likley choose the port as is designate thingy; so provided the current root switch priority is LOWER numerically than the new switch, because the lower the priority number of a BPDU means a higher priority ie priority 1 is higher than priority 2 etc., then all should be good.

Hope that helps.

Oh - and btw its usually not the best implementation to have an "edge" switch as a STP root (except for extenuating circumstances), check the cisco documentation for best practice.

EDIT : to answer the second question - bit tough to answer quickly this, this will happen because STP is running and will not assume that ONE connection to a neighbouring switch (or switches for that matter) is in existance. If two or more links exist then the one with the lower numerical priority (which includes link cost etc.) will be designated the root, if there is only one link or port then STP will select that sole port as there's no competition. Tough to explain quickly but I hope that clears it up.

- Sudo.

Message was edited by: Pseudopath

Scott_O'Brien Thu, 09/23/2010 - 18:54

Hi Carl,

if you plug in another switch into your root switch (lets call the current root switch A), if this other switch lets say switch B has a lower priority, switch B will now become the root bridge as it has a lower priority ( thus giving it a Higher priority). once switch B become the Root ( after the election process usally takes around 50 seconds) then all the ports on the Root bridge go to designated port state ( root port never have a blocking port unless you loop it back on its self). then the first switch ( switch A) as there is only one port going back to the root it will place this port as a root port.

carl_townshend Fri, 09/24/2010 - 05:40

Hi There

thanks for the replies, I know however about the priority of a root and how that works, we had an issue yesterday where both switches had same priority but the edge switch went to blocking for 30 secs.

My question was more around the edge switch to be honest, by default if the edge switch with a single link top to the core sees a better bpdu on the one link, will it still initially put the port the blocking whilst it relearns the best bpdu along its root port ?


Alan Taylor Fri, 09/24/2010 - 05:56

Hi Carl,

Can you provide some details of the topology in question please, that should help us better answer you question?


- Sudo.

jasonfmic Fri, 09/24/2010 - 06:44

If I understand you correctly, you had an existing root switch with an attached edge switch. You plugged a new edge switch into the root that happened to have the same bridge priority (I'm guessing with a lower MAC ) to the root switch. If we are talking about STP and not RSTP, when you did this you forced an election of a new root bridge. In STP, all switches in a topology must agree on a root bridge. Even though your existing edge switch only had one uplink and the new root bridge was in the same direction, outages can occur because all the switches need to agree on the new root and then ensure they still have the best path to the new root.

carl_townshend Fri, 09/24/2010 - 07:09

Thanks for the reply, that is what I wanted to know really, about why stp blocked the link

even though there was only 1 path from the edge switches point of view. So as a protection feature of stp, I gather it will still block the port for 30 seconds then begin forward using the new root bpdu's

can you confirm ?

jasonfmic Fri, 09/24/2010 - 08:11

A port is actually only blocking when the switch powers up, but ports will go back through listening and learning while the spanning tree is recalculated and in both the listening and learning states data frames are discarded. Only when ports move into the forwarding state are data frames transmitted.


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