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.

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.

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?