BGP advertisements identify reachable address prefixes and carry path attributes that help each AS apply its own routing policy and select routes.
External BGP exchanges routes between ASes; internal BGP distributes routes inside an AS. Peers establish a TCP session and then send incremental updates. Selection is policy-driven: the chosen path is not necessarily the geographically shortest, least congested, or most secure path.
Key points
Route informationNetwork Layer Reachability Information identifies prefixes, while attributes such as AS_PATH support selection and loop detection.
Routing policyOperators control which routes they accept, prefer, and advertise according to business, engineering, and security requirements.
Operational protectionPrefix and AS-path filters, maximum-prefix limits, controlled peering, session protection, monitoring, and change discipline reduce avoidable failures and abuse.
Important limitationBase BGP does not establish that every announcement is authorized or operationally correct. Resource Public Key Infrastructure (RPKI) route-origin validation can check whether an origin AS is authorized for a prefix, but it does not validate the complete AS path or prevent every route leak.