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CNPA: Routing Fundamentals

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Routing Fundamentals

This domain checks whether you can read a routing table and predict forwarding: longest match, administrative distance, metric, static versus dynamic routing, and the basic behaviour of RIP, OSPF, EIGRP and BGP.

Selection order

  1. Longest prefix match wins outright, regardless of AD or metric.
  2. Among routes to the same prefix, lowest administrative distance wins.
  3. Among equal AD and prefix, lowest metric wins. Default ADs: connected 0, static 1, eBGP 20, EIGRP 90, OSPF 110, RIP 120, iBGP 200.

Protocol profiles

  • RIP: distance vector, hop count, maximum 15 hops (16 means unreachable), updates every 30 seconds, split horizon and route poisoning limit loops.
  • OSPF: link-state, Dijkstra SPF, cost = reference bandwidth (default 100 Mbps) divided by interface bandwidth, so 1 Gbps and 10 Gbps both cost 1 unless auto-cost reference-bandwidth is raised. Areas, ABR/ASBR roles, DR/BDR election on broadcast segments.
  • EIGRP: DUAL, composite metric built from bandwidth and delay, feasible successor gives an instant backup, AD 90 internal and 170 external.
  • BGP: path vector between autonomous systems; AS_PATH and local preference drive selection; used for internet multihoming.

Static routes can specify a next hop, an exit interface or both (fully specified). A floating static sets a higher AD so it installs only when the primary route disappears.

Worked example

A router holds 0.0.0.0/0 via the ISP, 10.0.0.0/8 via R2, 10.1.0.0/16 via R3, and 10.1.2.0/24 learned by OSPF via R4, plus a floating static for 10.1.2.0/24 with AD 200. A packet to 10.1.2.55 matches /8, /16 and /24, so the /24 wins. Between the two /24 entries OSPF (110) beats the static (200), so traffic goes to R4 while OSPF is up. If OSPF fails, the static becomes the only /24 and installs. A packet to 10.9.9.9 matches only 10.0.0.0/8 and goes to R2, even though a default route exists.

Traps

  • Believing "lowest AD wins" globally; a /24 with AD 120 beats a /8 with AD 1.
  • Forgetting the default route applies only when no more specific route matches.
  • Assuming OSPF cost reflects real bandwidth once links exceed 100 Mbps.
  • Building a floating static with a lower AD than the primary, so it never floats.
  • Designing RIP networks beyond 15 hops, where destinations appear unreachable.