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Routing — Static, OSPF & BGP
When two routes to the same prefix compete, the lower administrative distance wins; ties break on metric.
OSPFlink-state · IGP
- Cost = 10⁸ / bandwidth (bps)
- Area 0 is the backbone
- DR/BDR elected on multi-access
- Multicast 224.0.0.5 / .6 · IP proto 89
BGPpath-vector · EGP
- Runs the internet between ASes
- eBGP AD 20 · iBGP AD 200 · TCP 179
- Picks path by AS-path length & policy
- Local pref / MED / weight tune it
RIPdistance-vector · legacy
- Metric = hop count, max 15
- 16 hops = unreachable
- Slow convergence, rarely used now