In the event of a disconnection between segments of the local network, automatic organization of a tunnel between them over the Internet?

Question

Computer networks

Hello fellow engineers!

Given:
LAN in three buildings that are connected in series by radio bridges.

There is a road between buildings No. 1 and No. 2, in connection with this, the use of a cable connection is not possible due to economic inexpediency. According to the norms, the cable must pass under the road, which requires large economic and time costs.

Scheme:

• Building No. 1 - 150 end devices.
  
• Building No. 2 - 50 end devices.
  
• Building No. 3 - 50 end devices.
  

Internet connection (VDSL 175 MBit/s), servers and most of the network are located in Building No. 1, it is not possible to shift the Internet connection and organize the network core to the Building No. 2 center.

Task:

• Building No. 3 needs another internet connection (VDSL 175 MBit/s).
  
• Network endpoints in Building 1 must access the Internet through the connection in Building 1.
  
• Network endpoints in building #2 and #3 must access the Internet through the connection in building #3.
  
• In the event of a break in the connection between buildings #1 and #2, it is necessary to automatically raise the tunnel to the network in building #1. Therefore, when the connection is restored, local traffic should go through the internal network again.
  
• All end nodes of the network must be on the same subnet.
  

Since the project is under development, the final solution can be tied to any vendor.

I would be extremely grateful for ideas, experience, best practices.

Answer 1

requirement:

All network end nodes must be on the same subnet

wherein:

the project is under development

So, while the project has not yet taken off, do not drive yourself into a corner with this requirement with the obligatory L2
. Immediately lay L3. There your Wishlist is implemented simply by any tunnel + ospf

Answer 2

All end nodes of the network must be on the same subnet.

Without this condition, it would be easier, but oh well, you have to abandon decisions on L3 and see what can be done on L2.
Try this: L2TP is raised all the time, and not just when the radio bridge is broken. But add STP, which will keep the tunnel in reserve until needed.
(Ugh, about VDSL speed buggy; wrote the answer early in the morning and took 175 MBit / s for 175 KBit / s).