HTTP/2 is the volution of the HTTP/1.1 protocol, originally developed by Google as SPDY it’s now a standard, and gives webserver faster responses to visitors.
First of all make sure your webserver is HTTP/2 enabled.
nginx -V 2>&1 | grep v2
it should show –with-http_v2_module. nginx/1.10.3 shipped with Ubuntu 16.04 has that enabled.
For Apache run
if it returns an error the module is not present. Apache2 shipped with Ubuntu 16.04 doesn’t have the module available, but if you install it from Ondrej repositories it should work. Beware that it can break the stability of the system.
Once the webserver is ready let’s go on ISPConfig control panel.
Go to System > Server Config > yourserver > Web > SSL Settings and enable the option Makes SPDY available.
After the checkbox is enabled you can enable HTTP/2 on your website.
Go to Sites > your site > SSL and at the bottom of the page you’ll have the option to enable SPDY there too.
I’m wondering why ISPConfig still calls it SPDY instead of HTTP/2, maybe because they’re waiting to merge #3868.
Image from sessionstack.