In order for OSPF to keep its topology current and up to date with any changes or failures, it has some important timers. Three timers I am going to talk about today are the hello interval timer, dead interval timer, and the retransmission timer.
Hello Timer- The hello timer is a timer that dictates how frequently OSPF routers will send hello packets. Hello packets serve many different OSPF processes but the periodic hello packets sent when the topology is fully converged is to maintain neighbor adjacencies. For broadcast and point-to-point networks the hello interval timer is usually 10 seconds by default. These periodic hello packets are sent by every router to inform directly connected neighbors that they are still active and reachable.
Dead Interval Timer- Hello packets serve as keep-alives because if a router does not receive a hello packet within a certain time interval that router will be declared unreachable by the directly connected neighbor. The interval that determines if a router is no longer active or reachable from not receiving a hello packet is the dead interval timer. This timer is usually 40 seconds by default on broadcast and point-to-point networks. For non-broadcast multi-access networks the default timer for hello is 30 seconds and dead interval timer is 120 seconds. The dead interval timer starts at the last received hello. From the last hello packet, if a router does not receive a hello packet within the dead interval timer the neighbor that didn’t send a hello packet will be declared unreachable and that router neighbor adjacency will reach a dead state. The hello and dead interval timers allow the OSPF topology to always be current and up to date on any changes such as a link going down, or a router failing.
Retransmission Timer- The third timer is a retransmission timer. This timer is more about reliability, as this is the timer that dictates when a LSA, carried inside an LSU should be retransmitted again. Whenever an LSU is sent in an area, the requesting router for that LSU must send an LSAck. An LSAck confirms to the sender that the LSU packet indeed reached its intended destination. If an LSAck is not sent within a retransmission timer, then the LSU will be flooded all over again. This allows all the routers in the OSPF area to be certain that the topology is identical, making OSPF a reliable protocol.
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