OSPF - configuring the shortest hello interval

For super fast convergence in ospf, you can change the ospf timers to the minimal setting. This will turn out to be a dead time of 1 second and a sub-second hello time.

Topology:

R1 --- R2

R1:

R1(config)#int s1/0
R1(config-if)#ip ospf dead-interval minimal hello-multiplier 3

Notice the hello interval is now 333 milliseconds:

R1#show ip ospf interface s1/0
Serial1/0 is up, line protocol is up
Internet Address 172.12.12.1/24, Area 0
Process ID 1, Router ID 1.1.1.1, Network Type POINT_TO_POINT, Cost: 64
Transmit Delay is 1 sec, State POINT_TO_POINT,
Timer intervals configured, Hello 333 msec, Dead 1, Wait 1, Retransmit 5
oob-resync timeout 40
Hello due in 217 msec
Supports Link-local Signaling (LLS)
Index 1/1, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 1
Last flood scan time is 0 msec, maximum is 4 msec
Neighbor Count is 0, Adjacent neighbor count is 0
Suppress hello for 0 neighbor(s)

R2:

R2(config)#int s1/0
R2(config-if)#ip ospf dead-interval ?
<1-65535> Seconds
minimal Set to 1 second

R2(config-if)#ip ospf dead-interval minimal ?
hello-multiplier Set multiplier for Hellos

R2(config-if)#ip ospf dead-interval minimal hello-multiplier ?
<3-20> Number of Hellos sent within 1 second

R2(config-if)#ip ospf dead-interval minimal hello-multiplier 3

R1#show ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 0 FULL/ - 704 msec 172.12.12.2 Serial1/0
R1#show ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 0 FULL/ - 936 msec 172.12.12.2 Serial1/0
R1#show ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 0 FULL/ - 636 msec 172.12.12.2 Serial1/0

As you can see with the "show ip ospf neighbor" command, the dead time counts down from 1 second and is reset every 333 msec when it receives a hello.

Changing the dead interval on R1 reverts the hello time back to the default:

R1(config)#int s1/0
R1(config-if)#ip ospf dead-interval 50

R1#show ip ospf int s1/0
Serial1/0 is up, line protocol is up
Internet Address 172.12.12.1/24, Area 0
Process ID 1, Router ID 1.1.1.1, Network Type POINT_TO_POINT, Cost: 64
Transmit Delay is 1 sec, State POINT_TO_POINT,
Timer intervals configured, Hello 10, Dead 50, Wait 50, Retransmit 5
oob-resync timeout 50
Hello due in 00:00:04
Supports Link-local Signaling (LLS)
Index 1/1, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 1
Last flood scan time is 0 msec, maximum is 4 msec
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 2.2.2.2
Suppress hello for 0 neighbor(s)

Pretty easy command, but it still helps labbing it, now I should never forget it...