A company has a site in Europe and another one in Mexico. They are interconnected by a private leased line. Each site is locally connected to an ISP (ISP-EU in Europe and ISP-MX in mexico). Border routers are respectively ER-EU in Europe and ER-MX in Mexico. Providers have allocated prefixes α in Europe and β in Mexico. Providers are doing in-gress filtering (i.e. a packet with a source address non allocated by a provider is discarded by the first provider router).
In the rest of the document, a host A (resp. B) will designate a host in Internet (outside company network) close to the ISP-MX (resp. ISP-EU) with a metric linked to BGP announcement. C (resp. D) will represent an equipment in the Mexican (resp. european) part of the company network.
Inside the site, an IGP routing protocol allows all routers to know all internal prefixes.