In a previous post (OSPF Hello Packets) I have mentioned that Hello packets are also used as a keepalive mechanism.  For this purpose, OSPF uses Hello and Dead Intervals.  These two parameters must match between neighbors to form the adjacency.

The default OSPF timers on Cisco routers depend on the media type of the interface; however it can be manually configured.

• For Broadcast Multi-access media and Point-to-Point, the default OSPF timers are 10 seconds for hello and 40 seconds for the dead timer.
• For Non-Broadcast (NBMA) the defaults are 30 seconds for hello and 120 seconds for the dead timer.

Configuring Hello and Dead Timers:

To configure the hello and dead timers use ip ospf hello-interval and ip ospf dead-interval interface commands.   The values are expressed in Seconds.

!
config t
interface e0/0
ip ospf hello-interval <1-65535>
ip ospf dead-interval <1-65535>
exit
!

OSPF Fast Hello Packets:

This OSPF feature allows sending hello packets at intervals of less than one second.

Yes!  Sub-Second convergence can be achieved with this feature.   OSPF Fast Hello is typically used in LAN segments to achieve fast convergence.    Please note that sending hello packets at sub-second intervals also means more processing power.  This may lead to high CPU usage and re-convergence triggered by congestion in some extreme cases.

The dead interval is set to minimal (1 second), and the hello-multiplier value is set to the number of hello packets that you want to send during that 1 second period.

When configured, the hello interval advertised in the hello packet is set to 0.  This means that the hello interval in the hello packet received by the neighbor is ignored. The hello multiplier does not need to be the same as long as at least one hello packet is sent within the configured dead interval.

Configuring Fast Hello Packets:

To configure Fast Hello use the ip ospf dead-interval minimal hello-multiplier <3-20> interface command.

!
config t
interface e0/0
ip ospf dead-interval minimal hello-multiplier <3-20>
exit
!

Always keep in mind that OSPF Hello and Dead timers must match to form neighbor adjacencies.   This also applies to OSPF Fast Hello dead-interval.

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