Cat3750 core to Meraki MS250 access switch stacks - STP issues

[GregCT]

Hi all.

Hi all - I had to install some new Meraki MS250 access switches over the weekend. They replaced some really old Cat 2960's. We decided to stack the Meraki switches and trunk them to the core with copper trunks spread across the stack using LACP etherchannel.

However, it looks like STP is blocking the secondary links, meaning I'm only getting 2x trunk links to stay up with the other 2x trunks in STP blocking mode. For info we have used 2x trunks in the top switch and 2x trunks in the bottom, to spread load and give us some hardware resilliency. Meraki stacks are a little different to how I'd expect a Cisco stack to behave, in that they are all still operating as individual switches, despite being physically stacked (we are using the Meraki stack cables here).

Coupled with the fact that Meraki STP is just weird - it doesnt fully support PVST+ which we were using on the core, and I don't really understand how to configure MST on the older 3750 switches properly. At present, the Meraki access switch is acting as STP root bridge for everything, which definitely isnt ideal.

My question is how should I be configuring STP in this scenario? I'm really struggling to find a definitive answer. It's a very simple layer 2 topology, consisting of a core switch stack, and the Meraki access layer switches trunked to the core. However STP is blocking the secondary trunks, which are all configured correctly for port aggregation, with identical port configs etc.

For the time being, I have changed the STP mode on the core to MST (from PVST+) however all this seemed to do was cause one of the Meraki switches to take the root bridge from the core.

Additionally, we have VLAN1 shutdown on the core - I read somewhere that Meraki MS uses VLAN1 to exchange STP status.

TL;DR: How do I get the core Cisco 3750 switch to become STP root in this scenario?

[NolanHerring]

I would start with allowing VLAN 1 across the trunk to your non-Meraki switch. I had to do the same, even though my cisco switches are not using VLAN 1 anywhere (strange i know). Also change the priority on the Meraki switch to something like 61440 for example.

[ww]

Did you config the switches as stack in the dashboard?  Or just only  connected  them  physically?

[PhilipDAth]

If STP is blocking the links then the LACP channel has not formed.

Either LACP is not configure correctly on the 3750 or is not configure correctly on the MS250s.

https://documentation.meraki.com/zGeneral_Administration/Tools_and_Troubleshooting/Link_Aggregation_... 

Potentially the Meraki switches are not correctly stacked together as well.

https://documentation.meraki.com/MS/Stacking/Switch_Stacks#Configuring_a_Physical_Switch_Stack 

[GregCT]

I can confirm the stack is correctly configured on dashboard as well as connected with cables. We are missing the 5th stack cable, but this isnt the problem, that just gives us less resilience. Funnily enough that cable is arriving today 🙂

As highlighted - these are the trunk links back to the core. LACP is configured correctly on all ports on the MS250's and the 3750's (mode active is enabled on all 4 ports), however STP is blocking the ports on Switch05. I'm pretty confident the issue is pure STP. If I bring up VLAN1, STP should start recognising the core and LACP will hopefully come up properly.