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?

Hi,

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

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?

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.

HTH,

-amit singh