Need help on configuring STP

Unanswered Question
Feb 25th, 2009

Hi Experts,

We have planned to put a redundant cores switch. We have configured HSRP for all the vlans. All the fiber uplinks from swithces placed at diferent floors are terminating on the primary switch. Now I want the backup fast ethernet copper links to be terminated to secondary switch. I fear if the situation might cause a loop for me. So wanted to know if any specific STP configuuration I need to do on the access level, primary and secondary switches. Attached is a diagram, which could give a better understanding of the situation. Let me know if any changes required.



I have this problem too.
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JamesLuther Thu, 02/26/2009 - 01:43


As long as you have spanning tree enabled on all your switches on every VLANN then you won't have a loop. It's also important to choose what type of spanning tree protocol to use (altough they should be interoperable with each other) ie PVST, RSTP or MST.

So given that you've got spanning tree configured on every switch you will be loop free. Next you just need to ensure that STP is optimised correctly. Do this by lowering the bridge priority on the two core swicthes. You will also want to align the placement of the root bridge with the active HSRP router for each VLAN.

OPTIONAL: If you want to loadbalance between uplinks then you will need to set SW1 to be the spanning tree root for half your VLANs and SW2 to be the spanning tree root for the other half. HSRP is then aligned to the root bridge for each VLAN. This normally done using SW1 for all odd vlans and SW2 for all even vlans.

Other than that try reading some of the Cisco docs on it




JamesLuther Thu, 02/26/2009 - 01:46


I put a link to STP above. Don't actually configure this (use RSTP, PVST or MST)!!!

I just put it up there as a reference for the basic theory.

arabindas Thu, 02/26/2009 - 02:00

Hi James,

Thank you for the information.

I have configuired rapid pvst on all the switches. (spanning-tree mode rapid-pvst).

Also I have made the primary switch 4506 as the root bridge for all the vlans. and secondary switch 3750 as the secondary root bridge for all vlans.

Hope this configuration works with the provided setup. I fear the network does not end up in loop.



Marwan ALshawi Thu, 02/26/2009 - 14:26

hi Arabinda

as James you can loadshare the traffic on both switches

why you put all the load on one switch now

just make for example even vlans use one switch as active hsrp and root bridge as well

and odd vlans use the other this way will be more practical and best practice as well

becuase having a passive switch just for backup not best way for your case


good luck

arabindas Thu, 02/26/2009 - 20:14


thank you. Surely I would try out what you have suggested.




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