Exchanging reachability information
BGP advertises how an IP prefix can be reached, attaching attributes such as AS_PATH and NEXT_HOP. Updates can also withdraw previously advertised reachability, allowing peers to maintain route candidates.
These are control-plane exchanges. User packets follow selected forwarding entries, so a BGP path is not an observation of every device traversed by an actual packet.
References: [1]
Policy participates in selection
A network assigns preferences under local policy and selects according to protocol rules. AS-path length is one input, while policy can express administrative goals and interconnection arrangements.
The selected route is not necessarily geographically closest or lowest-latency in a particular measurement. Different networks can have different preferences, making interdomain routing a coordination problem among operators.
References: [1]
Filtering and route leaks
Borders need policies for which prefixes to accept or advertise onward. RFC 7454 discusses prefix filtering as protection against propagating misconfiguration.
A route leak propagates learned announcements beyond their intended policy scope. It can redirect traffic, overload a path or create a black hole; syntactically valid updates over a live session do not establish correct scope.
RPKI origin validation is a scoped check. A ROA authorizes an AS to originate a prefix within specified length bounds; it does not validate the full AS path or detect every route leak. Validation state also differs from policy action: RFC 8481 prohibits automatic policy application without operator configuration. Valid, Invalid and NotFound must therefore be interpreted with authorization coverage and local policy, not as complete verdicts on path security.