I agree with Georg that bridging the sites together is possible but not recommended.
There is an interesting problem in the solution that Georg suggested. IP routing is enabled on Cisco routers by default. So if there is an IP address on the Ethernet interfaces then the router will be routing IP on those interfaces and bridging any other protocol that happens to be present. So IP from the Ethernet would not be bridged over the serial. To make the solution that Georg suggests work it would be necessary to add to the configuration the command "no ip routing". If the original post does not want to disable IP routing then it would be necessary to configure Integrated Rouging and Bridging. This would involve removing the IP address from the Ethernet interface, configuring a BVI, assigning the IP address to the BVI, and configuring IRB to process IP.
I thought a better diagram would help. My objectives where 1) To make connection R3-R4 one subnet with no devices in it other than what you see in the diagram. 2) To have ISP1 and ISP2 redundant and load balanced. 3) To protect the private Traffic R1-R2 by exposing only layer 2 info with bridging and vlan trunking.
I have looked at the new diagram. When you say: "1) To make connection R3-R4 one subnet" are you talking about the T1 between R3 and R4? making that one subnet is pretty easy and all you need to do is be sure that ip routing is enabled and assign addresses in a unique subnet to the serial interfaces.
2) To have ISP1 and ISP2 redundant and load balanced is possible depending on some things that you have not given information about. Probably the most direct solution is running BGP, but this objective can be met with other possibilities such as static and floating static routes.
I am not sure what your objective is in 3). I am not sure what protection you are trying to get by bridging. As Georg and I have pointed out bridging accross the WAN is usually not advised and makes the configuration more complex. Given the diagram I believe that just a straight bridging solution (no ip routing and configuration of bridge groups on each interface) will not work and you would need Integrated Routing and Bridging. And that address space would get exposed, so I am not sure where there is any protection. Perhaps you can help us understand better what your environment is and what your objectives are and perhaps we can suggest solutions that would work.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...