BGP

The Internet is not one network. It is a network of networks, each run under its own policy. Border Gateway Protocol, or BGP, is how those networks exchange reachability information.

BGP answers a practical question: which neighboring network should receive traffic for a destination prefix? It carries possible paths and enough attributes for each network to apply its own policy. That makes BGP different from an interior routing protocol. An interior protocol usually seeks efficient paths inside one administration. BGP coordinates paths between administrations that may have different technical and business goals.

The core mental model

Think of a BGP route as a claim with context:

I can reach this IP prefix through this path, and here are attributes you can use when deciding what to do with the claim.

A BGP speaker sends that claim to a configured peer. The receiving speaker applies import policy. It rejects or accepts the route, compares accepted routes, and selects a preferred route. It may then apply export policy and advertise a route to other peers.

That flow matters more than any vendor command:

peer advertisement
      ↓
import policy
      ↓
candidate routes
      ↓
decision process
      ↓
selected route
      ↓
export policy
      ↓
advertisement to another peer

BGP does not discover peers automatically. Operators configure the neighbor relationship and the policy around it. Modern default-reject behavior requires explicit import and export policy before an external BGP session can use or advertise routes.

BGP

The core mental model

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