Cisco Gigastack

Unanswered Question
Jul 17th, 2007

Hi All,

question regarding gigastack on a 2900 series switch... i've got a stack with 7 members - i need to add an additional switch to the loop - they are all daisy chained together, with a redundant link.

What kind of interruption will there be when adding the new switch - will it cause a spanning-tree type topology change amongst the stack members?

Thanks in advance.

I have this problem too.
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royalblues Wed, 07/18/2007 - 12:33

If adding the switch would cause a root bridge selection then there will be normal interuuption of about 50seconds (spanning tree convergence)

If you have enabled CSUF (cross stack uplink fast, the interruption could be minimal

Have a look at this link

HTH, rate if it does


debs123uk Wed, 07/25/2007 - 05:59

Thanks for the reply. We run five instances of MST for spanning tree, and have a defined root switch for every vlan - my query was more as to whether there is a kind of separate spanning tree instance, used internally only by the stack members? or whether when adding the new switch it will just find its way to the root through the stack, only blocking its own redundant link?

Amit Singh Wed, 07/25/2007 - 06:38


As you have configured the root switch for every vlan, when this new switch will come to your stack and it has all the vlans, it will find the root bridge for all the vlans on it own. However, its not sure whether it will block its own port or not as it depends on various factor and the calculation starts right from the TOP i.e for the root bridge.

STP uses the following order sequence to decide on its parameters

Step 1. Lowest Root BID

Step 2. Lowest Path Cost to Root Bridge

Step 3. Lowest Sender BID

Step 4. Lowest Port ID

HTH,Please rate if it does.

-amit singh

debs123uk Wed, 07/25/2007 - 07:08

Thanks for the info ... my concern really is to what effect adding the additional switch would have on the other stack members .. as the root is pre configured and isnt a member of the stack, am i right in saying there will be no topology change as such ?

the only stp impact will be on the new switch and the two switches that will be connected to it, via gigastack, one of which will become the redundant link?

Amit Singh Wed, 07/25/2007 - 07:16

There will be an impact on the whole STP domain as there will be TCN's flowing across the switches to the root bridge but the impact will be minimal as the new switch will accept the root bridge on the network. I would suggest to probably introduce the new switch after late hours. Also, check for VTP config, if you are running the VTP across the network.


-amit singh


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