Downloading http://cdn-fastly.deb.debian.org/debian/dists/stretch-backports/InRelease...
Success downloading http://cdn-fastly.deb.debian.org/debian/dists/stretch-backports/InRelease
gpgv:  RSA key ID 22F3D138
gpgv: Good signature from "Debian Archive Automatic Signing Key (10/buster) <ftpmaster@debian.org>"
gpgv:  RSA key ID 386FA1D9
gpgv: Good signature from "Debian Archive Automatic Signing Key (11/bullseye) <ftpmaster@debian.org>"

Mirror [mirror9]: http://cdn-fastly.deb.debian.org/debian/ stretch-backports successfully added.
You can run 'aptly mirror update mirror9' to download repository contents.