Routing Protocols

Distance-Vector · Link-State · BGP

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How routers build their forwarding tables automatically — from distance-vector rumors and Bellman-Ford, to link-state flooding and Dijkstra, to BGP path-vector routing across the internet's autonomous systems.

By the end, you can

Distinguish forwarding (fast, per-packet) from routing (slow, table-building) and explain what a routing table maps.
Explain the Bellman-Ford rule used in distance-vector routing and trace how distances propagate hop by hop.
Describe the count-to-infinity problem and name the four mitigations (cap, split horizon, poison reverse, hold-down).
Explain how link-state routing floods LSAs, builds a complete topology map, and runs Dijkstra — and why it converges faster than distance-vector.
Identify OSPF and IS-IS as link-state protocols and RIP as distance-vector.
Define autonomous system and AS number, and explain why BGP is needed between ASes.
Trace how the AS_PATH grows as a BGP advertisement propagates, and explain how loop prevention works.
Explain why BGP may choose a longer path over a shorter one due to LOCAL_PREF and business policy.
Distinguish peering from transit in inter-domain routing.
Identify and describe the four routing failure modes: convergence delay, route flapping, black hole, and route hijack.

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