BGP is how separate networks tell each other which addresses they can reach. Every internet provider runs it, and organizations that want to control their own internet presence — using their own address space across more than one provider — run it too.
Provider independence. With your own address space and an autonomous system number, you can announce your addresses through two providers at once. If one fails, traffic reaches you through the other without renumbering anything. Without BGP, changing or losing a provider means changing addresses.
Address space and an AS number from a regional registry, equipment capable of holding a full internet routing table if you take full routes, and somebody who understands routing policy. It is not exotic, but it is a commitment: a BGP misconfiguration can take your organization off the internet or, historically, disrupt other networks.
You can accept the entire internet routing table, a filtered subset, or simply a default route pointing at your provider. Most single-provider deployments need only a default; multi-homed networks generally take partial or full routes to make sensible path decisions. The choice affects how much memory your router needs.
Three failures account for most BGP incidents on enterprise edges: accepting a full table on a router without the memory to hold it, announcing prefixes without filters and leaking a transit provider's routes, and treating a second session to the same provider as redundancy when it shares the same physical path. Prefix filtering, RPKI or IRR registration, and a clear policy on what is announced to whom prevent nearly all of them.
BGP is the mechanism behind genuine provider redundancy. If your resilience plan is "we have two internet connections", BGP with your own address space is usually what turns that from two bills into an actual failover.
BGP is one of the specifications these comparisons turn on.
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