User/Mutual internet backup between two small ISP

Motivation

Assume two small ISP which operate in surrounding area and have own physical link between them. Both network use dynamic routing OSPF with own backbone and have their own upstream internet connection. As they operate in different area they don't compete to each other and so they have settle on mutual supplying backup internet connection for case of failure of their main link or part of their network.

Realization

Setup secondary default gateway

As both network use OSPF routing protocol they can get advantage of automatic propagation of multiple default gateways with defined priority. So they have to inject each other primary default gateway as secondary default gateway to own routing table.