OSPF Timers

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The correct and successful operation of OSPF requires the use of several timers.

🔹 The Hello interval (HelloInterval) is the length of time, in seconds, between Hello packets that the router sends on the interface. Command: ip ospf hello-interval; the default value depends on the OSPF network type.

🔹 The Dead interval (RouterDeadInterval) is the number of seconds before the router’s neighbors will declare it down, when they stop hearing the router’s Hello packets. Command: ip ospf dead-interval; the default value depends on the OSPF network type (typically 4 x HelloInterval).

🔹 The Wait timer defines the maximum length of the Waiting state and the value is equal to the Dead interval. In the Waiting state, the router is trying to determine the identity of the (Backup) Designated Router. No own command; the value is derived from RouterDeadInterval.

🔸 The Retransmit interval (RxmtInterval) is the number of seconds between LSA retransmissions. It is also used when retransmitting Database Description and Link State Request packets. Command: ip ospf retransmit-interval; the default value is 5 seconds.

🔸 The Transmit delay is the estimated time required to send a Link State Update packet over the interface. The Link State Advertisements contained in the Link State Update packet will have their ages incremented by this amount before transmission. This value should take into account the transmission and propagation delays for the interface. Command: ip ospf transmit-delay; the default value is 1 second.

The Hello, Dead and Wait timers (🔹) have to do with discovering, building and maintaining OSPF neighbor relationships. The Retransmit interval and Transmit delay (🔸) relate to propagating OSPF database information.