I had a strange issue with one of my branch offices, where they would lose access to local resources and external Internet sites whenever our Site-to-Site VPN with the head office went down.
I spent around 3 hours troubleshooting this issue, desperately looking for a logical cause. It wasn’t until I paid closer attention to the DNS settings that were being received from the DHCP server did I notice that the primary DNS nameserver was a legacy domain controller within the branch office that no longer existed, and the secondary DNS was a domain controller in our head office, across the VPN.
When the VPN link went down, the clients had no resolvable DNS servers, and thus couldn’t access anything except by direct IP.
When I discovered this, it was a quick fix that brought services back online promptly.
Unfortunately it is all too often that I dive into a problem looking for a cause that is complex without seeing the simple issue right in front of me. I need to learn to be a little more methodical in my problem solving, and start with Layer 1 first.