Symptoms
Upgrade of Mac OS from Catalina to Big Sur caused my Microsoft Remote Desktop client to suffer from constant drop outs and reconnects every few seconds. I was only able to work on the remote machine for maybe 10 seconds before the next reconnection.
Diagnosis
First of all, ensure that you have enabled the System Extension that was re-prompted after the Big Sur upgrade (dependant upon the Cisco Anyconnect version you currently installed). If confirmed, when you look at the System preferences for Network and see that the Cisco Anyconnect Socket filter is closing and reopening at the same time as the Remote Desktop disconnects. If so, then you have the same issue that I had.
Solution
Download the Cisco Anyconnect VPN client. Minimum version for Big Sur is 4.9. 04xxx which allows the System Extension framework in Mac OS 11 (Big Sur).
You will need to uninstall the previous version first by going into Finder, Applications, Cisco, then double click on Uninstall Cisco Anyconnect VPN.
Once uninstalled, install the downloaded client. Once done, you will be prompted to allow the system extension. Ensure that you do (this will require administrative privileges). This should now be stable and you will not encounter the dropouts as described above.
