What Is OSPF? Open Shortest Path First

OSPF (Open Shortest Path First, RFC 2328 v2, RFC 5340 v3) is the most widely deployed IGP (Interior Gateway Protocol) in enterprise networks. It is a link-state protocol based on the Dijkstra algorithm and scales through areas. It provides fast convergence (<1s with BFD).

How It Works

• Each router advertises the state of its links (LSAs)
• The LSDB is synchronized within an area
• Dijkstra calculates the shortest path
• Hello intervals: 10s on LAN, 30s on WAN

Areas

• Area 0 (backbone) is mandatory
• Regular: all LSAs
• Stub: no LSA 5 (external routes)
• Totally stubby: default route only
• NSSA: stub area with local external routes

Fast Convergence

• BFD: <1s instead of the default 40s
• NSF/SSO on HA routers
• LFA (Loop-Free Alternates)

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