I want to upgrade our 5.1.2 2xvSZ+2xvDP to 5.2.1 latest.
But I found from SZ-5.2.1-UpgradeGuide-RevA-20200731.pdf on page 23 this note:
"For remote APs connected over a VPN, the tunnel MTU must be reduced to 1400 (acceptable range is: 850 through 1500) to allow the configuration after upgrade. If there are many WLANs defined the MTU should be reduced further."
I checked quickly that our IPSEC tunnels for remote locations have MTU 1422 in the central VPN device and that this can't be changed with the current software version. There are reasons why the upgrade of the VPN-device wouldn't be a good idea at the moment. It has the latest software in its line so it isn't any ancient device though. Also, I didn't see such a note in the upgrade guide for 5.1.2 so this is a new note not just a general suggestion.