How OSPF Hello Packets Filter Neighbor Relationships Before Adjacency Forms

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An OSPF Hello packet does more than act as a keepalive sent every 10 seconds — it functions as a strict filter that determines whether two routers can begin a neighbor relationship at all. Before any adjacency or database exchange can occur, four fields in the received Hello packet must exactly match the local interface configuration: area ID, netmask, hello interval, and dead interval. If any of these fields differ, the relationship silently never starts, with no error sent to the remote router and often no visible log entry. This behavior explains a common troubleshooting scenario where two routers on the same segment both send Hellos yet never form an adjacency. A free educational series called 'Protocol in Code' explores this logic through a compact Python implementation available on GitHub.

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