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Max spanning-tree instance when merging networks.


I have been tasked with migrating 24 access switches from their current distribution switches, 2x stack 3750's to a their new 4500 distribution switches.

Ideally with no downtime. My plan for this was to connect the 4500's to the 3750's and create replica vlans on the 4500's therefore spanning the L2 broadcast domain across both sets of switches.

Each one of the vlans that had been created on the 4500's would have the STP bridge priority set to 4096 and 8192. When one of the uplinks from the access switches to the old 3750 stack is connected to the 4500's, as I understand this should have a stp cost of 4 and move the old uplink into

discarding therefor passing traffic across the new link. This would then allow me to disconnect the old link and connect it to the second 4500 as the L2 traffic is spanned between all switches. Not ideal but the only way I can see this working.

Once each of these switches have been moved, the SVI's and static routes will be moves from the 3750 stack to the 4500s.

The problem I have is that I've ran out of the available STP instances I can use, these 3750's all seem to be at 128 already so as far as I know and please confirm or correct me, is that if a loop was to be formed when the 128 allocation is hit this would indeed cause loop in the network.

Here is the other problem, I have a total of in use vlans to be spanned of 700. I'm unaware how many instances of stp the 4500's can deal with however I can bet it's not 700. 3000 logical interfaces I believe is the maximum I can see online with regards to how many stp instances the 4500's can use.

Can anyone think of another way to do this? Is the only real safe way to do this to 'shutdown' the old uplink and 'no shut'  the new uplink?

Any help or comments or corrections appreciated.


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Max spanning-tree instance when merging networks.


if you go over the amount of 128 instances there will not be a loop. The port will be blocked at the other end of the link of the 3750 (In our case this is a couple of 6509-E's) as they do handle this amount of stp instances, but there are some disadvantages.

We have a workaround for that, we use allowed vlans on the trunk uplinks (both on the 3750- and the 6509 side) so that we only allow the vlans we need towards each 3750's side. Not sure if this is a solution for you though. This is very time consuming as for configuration but it works because you can manipulate the amount of stp instances by working with allowed vlans.  We have around 180 vlans but with the allowed vlans feature we managed to get only around 15 stp instances per 3750 stack for different vlans


Max spanning-tree instance when merging networks.

As far as I understand u correct udon't need all VLANs on the 3750, so it should be possible to limit the Trunk between the two distribution switches to just the necessay VLANs. The 3750 will get a problem as soon as u configure more than 128 VLANs on it and STP is enabled for all of them.

So both sides as vtp mode transparent should work, I would not configure a server/client enviroment with one VTP domain over both distributions.

Once I disabled STP in one case for a time I had to handle more then 128 but I really don't like that and it will make trouble in your case with two Uplinks to both distributions.

did that help?

Max spanning-tree instance when merging networks.

Thanks for the replies. They are both appreciated.

The problem is that as the SVI interfaces exist on one of the 3750 switch stacks so the ~1000 vlans will have to be spanned through all switches so that the access switches can route via these SVI interfaces.

This means that the allowed vlan list will still be significant, more than 128. The solution to this is to remove STP from the problem by shutting down the old uplink before raising the new uplin ensuring rstp is running to it should move to a  forwarding state in around ~6 seconds.

Once again thank you for your input, most helpful.

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