Configuring L2TP VPN on Zyxel USG 60 for Chromebook / ChomeOS compatibility

For a couple of years I have had no problem connecting to my office VPN from my Chromebooks. Prior to departing for a long trip I decided to just double-check that all was well. Unfortunately, it wasn't. I could no longer connect to the VPN from either of my Chromebooks, even though I still connected successfully from Windows and from Android. I assumed that Google changed something, but as it turns out, it was probably a change that I made in the USG that caused the problem.

After months of frustration, I came upon an blog post that detailed the ipsec VPN configuration settings on a Fortigate router needed to work with the Chromebook's built-in VPN capability.

https://dbssolutions.freshdesk.com/support/solutions/articles/196790-how-to-configure-a-fortigate-ipsec-vpn-for-google?_utm_source=1-2-2

L2TP depends upon underlying ipsec protocols so I compared mine on the USG to those documented in the post above.

Looking at the IPSEC Phase 2 configuration, I saw that the policies did not match those I was using. Once I changed them to match those in the article, the Chromebook connected no problem. The results suggest that the Chromebook doesn't like SHA authentication. A while back I had changed a Zyxel default policy that specified MD5 authentication to SHA. Once I re-created a policy that uses MD5 authentication instead, the VPN link was established successfully and quickly. Here are my new working settings: