I have a customer that owns 4 class C subnets. We are beginning a project to multihome them between two providers. The design we want to accomplish is to have 3 of the subnet to utilize provider A for inbound and outbound service while the fourth remaining subnet uses provider B for both inbound and outbound service. In the event of a provider failure all subnets need to switch to the remaining provider.
Question is: In thinking this through, I think I need two AS numbers if I want to insure both inbound and outbound traffic are isolated to a single provider under normal operation. Can some enlighten me if I can get it done using just a single AS number.