BGP is the only exterior routing protocol in widespread use. BGP supports policy-based routing, which allows you to define organizational or political reasons for choosing a route. The routing metrics used within different routing domains cannot be directly compared. You cannot know exactly how the metrics are determined within another routing domain. You may not even know what interior routing protocols they use. Instead of trusting the technical process used to select the routes, you trust the organization that advertises the routes. You may do this because of the reputation of the organization or because you have a business agreement with them to trust their routes. This is the basis of policy-based routing.
BGP is a path-vector protocol. The path vector provides an end-to-end list of every routing domain along the route, which allows you to decide whether or not you trust the advertisements that come from those domains.
You probably will not run BGP unless you are told to run it by your ISP. Exterior protocols are primarily used externally, and for them to be used successfully, both parties must agree to use the protocol. If your ISP requires you to use BGP, you can be sure your ISP knows how it should be configured. The protocols that you use on your own network are the interior routing protocols.
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