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Network Layer: Control Plane

How routers decide where traffic should go. The module starts with what a routing table is and when it converges, then covers the two algorithm families (link-state and distance-vector), how the Internet splits into autonomous systems, and OSPF inside one. BGP takes over between them, driven by business policy and open to hijacks and leaks. The module ends with the diagnostic tools that read the network back: ICMP, ping and traceroute.

What you will be able to answer​

  • What is the difference between the control plane and the data plane, and what does convergence mean after a link fails?
  • How do Dijkstra (link-state) and Bellman-Ford (distance-vector) differ, and why does distance-vector suffer count-to-infinity?
  • What is an autonomous system, and why does the Internet use one protocol inside an AS and another between them?
  • How do two OSPF routers become adjacent, what is in the LSDB, and why do areas exist?
  • How does a BGP session start, what does AS-PATH do, and how do eBGP and iBGP differ?
  • Why does BGP prefer local preference over AS-PATH length, and how do business relationships shape routes?
  • How does a BGP hijack or leak happen, and what does RPKI actually prevent?
  • What do ICMP Time Exceeded and Echo messages carry, and how does Path MTU Discovery depend on them?
  • How does traceroute use TTL to map a path, and why can its output mislead?
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