Stacking MS120 switches

[JordanCN]

I have a site that already has 3 of the MS120-48P (no stacking ports in the back).  Can 3 of the MS120 units be cabled as a stack so if one of the switches dies, the other two can still communicate?  Or do I have to go get some MS2xx series switches?  

[Ryan_Miles · Accepted solution]

Yes, you can use regular LAN ports to interconnect the three switches together. This of course will create a loop and spanning tree will block port/ports where needed. I assume your diagram implies creating 4 port LAGs on each switch?

Of course this won't be a stack in the same way a 2xx/3xx series stack with the dedicated stacking port. But, it achieves your redundancy concern if a single switch fails.

[JordanCN]

Yes, I would be forming a 4 port LAG between the switches.

We do have RSTP enabled so will the switches automatically change over to the available path if one dies?

Also, these three switches are not the only ones on the LAN.  Other switches are branched off from these.  Would you suggest I make changes to the Priority settings to ensure that these three have the higher priorities?

[Bruce]

Correct, if one fails STP will reconverge onto the remaining working links. If this is the core of your network then I would definitely change the STP priority to make one of the switches the STP root, and the other two the likely second choices.

[PhilipDAth]

While this should work on paper, I wouldn't recommend this design.  Ideally, strive for a loop-free design.  My personal guess - you have 1 to 2 outages per year caused by the redundancy of this design, and probably no outages if you make one switch the core and simply cable both other switches back to only the core.

My personal experience is mal-functions in spanning tree make the network more susceptible to outages even though it adds more redundancy.

As you indicate, you could get a pair of MS210-24P switches and hardware stack them, and dedicate them to being core switches.