OSPF Timers (OSPF Intervals)

Routing protocol use Timers (Intervals) to keep all routers in a domain updated.

Through the use of these timers, a routing protocol is able to maintain a stable neighbor relationship and ensure routes are propagated correctly.

OSPF routing devices constantly track the status of their neighbors by sending and receiving Hello Packets to confirm whether each neighbor still is functioning normally. We can use these OSPF Intervals (OSPF Timers) to adjust the network convergence time in an OSPF network.

Types of OSPF Timers:

OSPF uses two types of OSPF Intervals (Timers):

1. HELLO Timer Tells the router after how long should it send the hello packet
2. DEAD Timer Tells the router how long should it wait for hello packets before it declares the neighbor dead

*There are some other OSPF Timers (OSPF Intervals) as well which might differ a bit from vendor to vendor. For example, Cisco uses these five types of OSPF Timers:

• Hello Timer: Interval time in seconds that a router sends an OSPF hello packet. On broadcast and point-to-point links, the default is 10 seconds. On NBMA, the default is 30 seconds.
• Dead Timer: Time in seconds to wait before declaring a neighbor dead. By default, the dead timer interval is four times the hello timer interval.
• Wait Timer: Timer interval that causes the interface to exit out of the wait period and select a DR on the network. This timer is always equal to the dead timer interval.
• Re-transmit Timer: Time to wait before retransmitting a DBD(database description) packet when it has not been acknowledged.
• Hello Due-in Timer: An OSPF hello packet is sent on this interface after this time.

Default values of OSPF Intervals:

OSPF Interval (Timer) values are different for different type of networks. We can change them manually as well according to our requirements.

Important Points about OSPF Intervals:

• The Hello and Dead timers must match to form a neighbor relationship in OSPF (ISIS & EIGRP can form even with mis-matched intervals)
• The Dead Timer is typically four times the amount of the Hello Timer to give time for packet loss/drops due to network issues and/or quality of service policies
• OSPF Hellos are sent to all OSPF Routers on the Multicast Address of 224.0.0.5

Open Shortest Path First (OSPF)