OSPF uses certain messages for the communication between the routers operating OSPF.
Hello message : These are keep-alive messages used for neighbor discovery /recovery. These are exchanged every 10 seconds. This includes the following information: Router I’d, Hello/dead interval, Area I’d, Router priority, DR and BDR IP address, authentication data.
Database Description (DBD) : It is the OSPF route of the router. This contains the topology of an AS or an area (routing domain).
Link state request (LSR) : When a router receives DBD, it compares it with its own DBD. If the DBD received has some more updates than its own DBD then LSR is being sent to its neighbor.
Link state update (LSU) : When a router receives LSR, it responds with an LSU message containing the details requested.
Link state acknowledgement : This provides reliability to the link-state exchange process. It is sent as the acknowledgement of LSU.
Link state advertisement (LSA) : It is an OSPF data packet that contains link-state routing information, shared only with the routers to which adjacency has been formed.
Note – Link State Advertisement and Link State Acknowledgement both are different messages.
Timers :
Hello timer : The interval in which the OSPF router sends a hello message on an interface. It is 10 seconds by default.
Dead timer : The interval in which the neighbor will be declared dead if it is not able to send the hello packet. It is 40 seconds by default. It is usually 4 times the hello interval but can be configured manually according to need.
