Routing & Switching

OSPF

Open Shortest Path First — the industry-standard link-state IGP. Four chapters build it up properly: link-state flooding and the LSDB, the neighbour state machine, the area system with its LSA types, and DR/BDR election with OSPF's network types.

read in order, or dip in: 4 chapters
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Contents

01OSPF Fundamentals: Link-State Routing and the LSDBOSPF is a link-state routing protocol: every router floods link-state advertisements describing its connections, so all routers in an area hold an identical topology database and independently compute shortest paths with Dijkstra's SPF algorithm. Cost, derived from interface bandwidth, is the metric, and OSPF converges by re-flooding only what changed.
02OSPF Neighbor States ExplainedTwo OSPF routers move through eight states on the way to adjacency: Down, Init, 2-Way, ExStart, Exchange, Loading and Full — with Attempt used only on manually configured NBMA neighbours. Full means the LSDBs are synchronised; a healthy pair of DROthers stopping at 2-Way is the one 'stuck' state that is actually correct.
03OSPF Areas and LSA TypesOSPF scales by dividing the domain into areas joined through backbone area 0: full topology detail stays inside each area, and Area Border Routers pass summarised reachability between them. Six LSA types do the work — router (1) and network (2) inside an area, summary (3) and ASBR-summary (4) between areas, external (5) for redistributed routes and NSSA-external (7) inside not-so-stubby areas.
04OSPF Network Types and DR/BDR ElectionOn multi-access networks OSPF elects a Designated Router and Backup Designated Router to stop n² adjacency explosion: all routers form full adjacencies only with the DR and BDR, and the DR originates the network LSA for the segment. Election prefers highest priority then highest router ID, is non-preemptive, and does not occur on point-to-point links.

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