Sharing one VLAN with two routers each one will route different subnet

Question

We are transitioning to new WAN connectivity. I have one VLAN (let's say ID 10). Currently there is one router R1 that has one private subnet and one public subnet on the VLAN 10 which goes through the network to different endpoints. Can I insert another router R2 into the network have the VLAN 10 joint it and move only the private subnet to the new route R2. So that now the R1 will route the public subnet and R2 will route the private subnet? Of course disabling the private's subnet gateway IP and R1.

Will it work? Or is there going to be a collision/problem with two routers sharing the same L2 VLAN albeit routing different subnets? I am not sure how it will behave. It's only a temporary solution until I am able to route the public subnet on R2 too.

Essentially I want to share the same one L2 VLAN with two L3 routers each of which will route one different subnet which were originally routed by one router.

Current State:

Transitional State:

End State:

EDIT: Included the pictures to better explain what I want to achieve

Answer 1

The problem is your handover subnet 77.5.6.20/30. It only allows two routers to attach (.21 and .22) - the ISP one and yours. No way to attach a third router.

Of course, you could attach R2 behind R1 within the 88.10.1.0/27 subnet. R1 would then route transparently and only R2 would NAT your private subnet(s).

As a side note, you really shouldn't run a public and a private subnet within a single VLAN - securitywise that's suboptimal, mildly put. Instead, you should separate the VLANs and consider a decent DMZ concept (an additional firewall between the public and the private VLANs with (next to) no connectivity from public to private). As it is, a comprimised public host could directly attack private nodes which are likely less protected.

Answer 2

No, this wont work sadly. A router sits between networks and routes traffic between the networks. All you appear to be doing is creating two default gateways for subnets and configuring them on different routers. Because everything is using the same vlan 10 there is not way for the two routers to communicate with each other.

If a PC with IP 172.18.10.55 wants to get to google (8.8.8.8) it will send its packet to its default gateway 172.18.10.1 (R2). But from then on R2 has no way to get to R1. The route from R2 to R1, technically, would be back down into the 172.18.10.0/24 network to a host with an IP address that is outside of that subnet. That doesnt work.

If you are unable to route traffic at the moment using R2 then it isnt functioning as a router and you need to wait until you can route before adding it to your network.