BGP Route Propagation

Path-vector routing, policy filters, and hijacks

A small autonomous-system graph announces a prefix. Watch the path propagate hop by hop, shaped by peer-vs-customer export rules and local-pref. Then declare a hijack from another AS and watch the lie spread along exactly the same policy rails.

origin AShijackerhop 0/8

RIB entries at hop 0

AS1— no route —

AS2— no route —

AS3— no route —

AS4— no route —

AS5— no route —

AS6— no route —

AS7— no route —

AS8— no route —

AS99len=1

AS10— no route —

Policy rule: a route learned from a peer or provider is not re-exported to another peer or provider — only to customers. This is why a Tier-1 does not carry a customer's routes to its own peers for free.

BGP does not find shortest paths. It finds paths consistent with each AS's business policy, and the internet is the fixed point of those choices.

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