OSPF immediate hello packet

When two Cisco OSPF routers initially form an adjacency, they automatically send out a hello packet. As soon as an OSPF router receives such a hello, it responds with what is known as an immediate hello, regardless of the hello timer and of the last hello sent.

A debug example from debug ip ospf packet can be seen here:

*Jul 22 08:05:04.930: OSPF-1 HELLO Gi1: Rcv hello from 2.2.2.2 area 0 12.12.12.2 *Jul 22 08:05:04.930: OSPF-1 HELLO Gi1: Send immediate hello to nbr 2.2.2.2, src address 12.12.12.2
  • The router received a hello and immediately (same timestamp) sent out a hello.
  • This is done to establish an adjacency as soon as possible rather than wait for the expiry of the hello timer, which by default for an Ethernet interface is 10 seconds.
  • This is a Cisco enhancement to OSPF and is not included in the RFC 2328 definition.

https://datatracker.ietf.org/doc/html/rfc2328

https://community.cisco.com/t5/routing/ospf-hello/td-p/1875572

https://forum.networklessons.com/t/ospf-hello-and-immediate-hello/14951/2?u=lagapides

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