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STP SF-300 and C2960 - Doesn't Work


     I have been having problem to set up STP among switches, as i whish it were. The topology is  this,


The way that all this is working is


i would like it working in this way


I've reviewed de Bridge ID so that, Switch A has lowest BI, and switches C and D have de same highest BI. Then,  the path cost between switch C and D has a cost higher than the link againts Switch B.

But i can´t block the link between Switch C and D.

The strange thinks that i saw, were

      > Swicth D has not BI and Address.

      > Switch C has as STP ROOT de BI of Switch D

I would be gratefull if you could helpme or leadme to fix this issue.




Two things to think about:

1. STP pathcost calculation method.  The 2960 and SB switches use different calculation methods (short vs long).  You may need to set 'spanning-tree pathcost method long' on the 2960's to make them use the same method as the SB switches.

2. The 2960 is probably doing PVST (per-vlan STP) and the SB switches are probably just running IEEE STP.  Note that the SB's STP is not per-vlan.  This means you only need to care about STP on VLAN 1.

You can optionally set both 2960 and SB switches to use Rapid Spanning Tree for faster convergence.  The SB switches don't do Rapid-PVST, but they will do Rapid IEEE.  It won't solve your problem but it would be a good idea.

To further look into this we may need the output from 'show span' on vlan 1, on all devices so we can see what switches are seeing what else and what costs are being used to calculate the roots and blocking ports.

Community Member


Thanks for your answer.

The key to fix the problem was the point 2. I had to propagate the vlan 1 to all device, in spite of it wouldn't be used. Then, i configured root priority command into the core switch and  Afterward, the SPT root was selected rightly.