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I have a local network of several subnets, let's call them subnet A and B. As expected, one of my clients within subnet A can ping other clients within both subnet A and subnet B. But as soon as I connect to a remote VPN, the client can no longer ping other clients only on subnet B (but can ping clients on subnet A).

When I use traceroute to figure out where its going, the first entry shows its going to the IP configured for the VPN network (no big surprise there).

So I'm guessing its either a problem with my VPN settings (using Tunnelblick/OpenVPN) OR a problem with my router (using EdgeLite Router).

But how does one go about troubleshooting this?

UPDATE (Nov 15, 2017 at 2:03pm MST)

@Zoredache thanks for your answer.

I found I can use 'netstat -r' to show the routing table. I sampled the table both w/ and w/o VPN connected, and got what I expected. When connected to the VPN, the only difference was new destination addresses added to the utun1 network interface. But I did notice something that was curious...

$ netstat -r
Routing tables

Internet:
Destination        Gateway            Flags        Refs      Use   Netif Expire
...    
10                 192.168.237.17     UGSc            2        0   utun1
...

What is '10' address here? Is that really all destination 10.xxx... are routed to the VPN server? How does this get into the routing table? And how to have it removed? Is this a VPN server-side thing? I suspect this is my problem here.

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how does one go about troubleshooting this

You start by looking at your route table. The route table defines where packets will be sent. When your VPN is connected your routes are different, so packets follow different paths. You need to make the routes that will send the traffic to through the correct gateways.

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