MS stack and MX

[nst1]

Currently I have two MS (in stack) and an MX I want to connect a second cable from the second MS is this possible?? because I already tried it and the network becomes unstable, that is, I start losing packets.

[Brash]

With the MS in a stack, they are essentially a single switch.
When you connect a second cable to the MX, you're inadvertently causing a loop in the network as you have 2 connections between the devices.

Because the MX does not run spanning-tree, it will pass BPDU's through between the two links. In theory, spanning-tree should detect this loop and disable (AKA spanning-tree block) one of the links.

Have a look at the following thread for more info.

Solved: more uplink from MS switch stack to MX firewall make switches unavailiable - The Meraki Comm...

[alemabrahao]

I have 2 pairs of MX with several ports connected in a pair of stacked switches and it has been working without any problem.

LACP & Link Aggregation
The MX does not run LACP or any link aggregation protocols. Connecting aggregated ports to the LAN of the MX is not supported; all connected ports should be un-aggregated. If multiple ports are connected to the MX from a single switch for redundancy, it is highly recommended that you run STP on that switch, to ensure that one of the redundant ports is safely put into a blocking state.

[KarstenI]

Here is a blog-post that explains a little bit about this problem:

https://cyber-fi.net/index.php/2022/03/13/how-to-connect-the-meraki-mx-to-ms-switches/