OSPF multiple OSPF nieghbours on layer 2 segment

RouterOS Forwarding Protocols
1

This may well be an OSPF design flaw by myself and I could potentially fix it but I wanted to ask if there is another way before I decide to run vlans over my router connections.

Basically I have R3 connected to R1 and R2 via an external providers fibre connection. The external provider has given me a VLAN to tag for accessing my interconnected sites.

So Site B contains R3, site A contains the switch, R1 & R2
My switch terminating the fibre has connections to both R1 & R2 untagged in the vlan that goes through to site B.

So my R3 sees neighbours R1 and R2 on the same interface.

I want to make R1 the primary route and R2 the backup for all routes.

How can I achieve this as interface cost in this case won’t work as I understand it.

2

Hi, sorry my english.
Maybe you can setup one instance for each connection between routers and set different metrics in ospf instances. Lower metrict have preference than bigger metric. Don´t forget to set redistribute other OSPF routes.

3

I understand how to set the metrics in multiple instances, but I am not sure if I understand if multiple OSPF instances would be the solution here/might cause more problems?