Why it matters

BGP issues cause the most famous internet outages (2021 Facebook, 2023 Cloudflare). Understanding it helps diagnose and design against these.

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The architecture

Autonomous System: a network under single administrative control. Has AS number.

BGP peering: ASes exchange routing info directly with peers.

BGP fundamentalsAS advertises prefixe.g. 10.0.0.0/8Peers propagatewith AS pathRoute selectionshortest AS pathBGP is policy-heavy: preferences, filters, communities influence route selection
BGP routing basics.
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How it works end to end

Path selection: BGP prefers shortest AS path, most-specific prefix, best MED, then tiebreakers.

Policies: filter what to advertise, prefer specific peers. Business-driven.

Route hijacking: malicious AS advertises prefix it doesn't own. RPKI helps prevent.

Outages: bad advertisements can cause internet-wide traffic redirection.