OSPF Overview
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OSPF is a link-state protocol: instead of trusting neighbors' summaries (distance-vector), every router floods a description of its own links, and each router independently computes shortest paths over the resulting shared map (the LSDB).
The essentials
| Property | Value |
|---|---|
| Type | Link-state IGP |
| Transport | IP protocol 89 (no TCP/UDP) |
| Metric | Cost = reference bandwidth / interface bandwidth |
| Multicast | 224.0.0.5 (all OSPF) · 224.0.0.6 (DR/BDR) |
| Hello / Dead | 10s / 40s on broadcast links (defaults) |
| Authentication | None / plaintext / MD5 / SHA (OSPFv2) |
Concepts you'll meet everywhere
- Router ID — 32-bit identity, chosen from explicit config, else highest loopback IP, else highest interface IP. Set it explicitly; mysterious adjacency problems love auto-picked RIDs.
- DR/BDR — on multi-access segments, routers elect a Designated Router so everyone syncs with one point instead of full-mesh flooding.
- Areas — LSDB scope boundaries that keep SPF small; see Areas.
Cost gotcha
The default reference bandwidth (100 Mbps on many platforms) makes every interface ≥100 Mbps cost 1 — a 10G and a 1G link look identical. Raise the reference bandwidth consistently on all routers.