Routers in an OSPF routing domain should first build up full relationships before starting to exchange LSAs and then produce the SPF tree. Two routers can become adjacent if they meet some requirements, including having the same OSPF Hello and Dead interval values inserted in the Hello packets they are exchanging.

OSPF uses many timers, including the Hello timer. The Hello timer is an interval timer that is triggered every HelloInterval seconds and instructs the router to send a Hello packet over a particular interface.

HelloInterval is announced in OSPF Hello messages (Figure 1), and represents the amount of time, in seconds, between two consecutive Hello packets sent over the interface. The OSPF Hello interval may not be the same on all router interfaces.

Figure 1 – OSPF hello packet sent by R2

Additionally, RouterDeadInterval or Dead interval in OSPF is announced in OSPF Hellos transmitted out an interface (Figure 2), and it represents the amount of time in seconds the router wait to receive a Hello packet from a neighbor before the router declares it down. If the router does not receive Hello packets from a neighbor within RouterDeadInterval seconds, the neighbor in question gets marked down.

Figure 2 – OSPF hello packet sent by R2

In the rest of this tutorial, we will be using the network topology in Figure 3.

Figure 3 – The network topology of an OSPF routing domain

Default OSPF Hello and Dead Interval Values

The default OSPF Hello and Dead interval values vary from one OSPF network type to another. The table below lists all OSPF network types along with their corresponding Hello and Dead interval values.

OSPF Network Type Default Hello Interval Default Dead Interval
Point-to-point 10 40
Point-to-multipoint 30 120
Point-to-multipoint non-broadcast 30 120
Broadcast 10 40
Non-broadcast 30 120

Notice that the default Dead interval is four times the default Hello interval.

To illustrate the data in the table, we configure our OSPF routing domain as follows:

Router Interface IP Address OSPF Network Type
R1 GigabitEthernet 0/2 10.0.12.1/24 Point-to-point
GigabitEthernet 0/4 10.0.14.1/24 Point-to-multipoint
R2 GigabitEthernet 0/1 10.0.12.2/24 Point-to-point
GigabitEthernet 0/3 10.0.23.2/24 Point-to-multipoint non-broadcast
GigabitEthernet 0/4 10.0.24.2/24 Broadcast
R3 GigabitEthernet 0/2 10.0.23.3/24 Point-to-multipoint non-broadcast
GigabitEthernet 0/4 10.0.34.3/24 Non-broadcast
R4 GigabitEthernet 0/1 10.0.14.4/24 Point-to-multipoint
GigabitEthernet 0/2 10.0.24.4/24 Broadcast
GigabitEthernet 0/3 10.0.34.4/24 Non-broadcast

Here are the configurations applied to the routers:

Router R1

Router(config)# hostname R1
R1(config)# 
R1(config)# interface GigabitEthernet0/2
R1(config-if)# ip address 10.0.12.1 255.255.255.0
R1(config-if)# ip ospf network point-to-point
R1(config-if)# no shutdown
R1(config-if)# 
R1(config-if)# interface GigabitEthernet0/4
R1(config-if)# ip address 10.0.14.1 255.255.255.0
R1(config-if)# ip ospf network point-to-multipoint
R1(config-if)# no shutdown
R1(config-if)# 
R1(config-if)# router ospf 1
R1(config-router)# router-id 1.1.1.1
R1(config-router)# network 10.0.12.1 0.0.0.0 area 0
R1(config-router)# network 10.0.14.1 0.0.0.0 area 0

Router R2

Router(config)# hostname R2
R2(config)# 
R2(config)# interface GigabitEthernet0/1
R2(config-if)# ip address 10.0.12.2 255.255.255.0
R2(config-if)# ip ospf network point-to-point
R2(config-if)# no shutdown
R2(config-if)# 
R2(config-if)# interface GigabitEthernet0/3
R2(config-if)# ip address 10.0.23.2 255.255.255.0
R2(config-if)# ip ospf network point-to-multipoint non-broadcast
R2(config-if)# no shutdown
R2(config-if)# 
R2(config-if)# interface GigabitEthernet0/4
R2(config-if)# ip address 10.0.24.2 255.255.255.0
R2(config-if)# ip ospf network broadcast
R2(config-if)# no shutdown
R2(config-if)# 
R2(config-if)# router ospf 1
R2(config-router)# router-id 2.2.2.2
R2(config-router)# neighbor 10.0.23.3
R2(config-router)# network 10.0.12.2 0.0.0.0 area 0
R2(config-router)# network 10.0.23.2 0.0.0.0 area 0
R2(config-router)# network 10.0.24.2 0.0.0.0 area 0

Router R3

Router(config)# hostname R3
R3(config)# 
R3(config)# interface GigabitEthernet0/2
R3(config-if)# ip address 10.0.23.3 255.255.255.0
R3(config-if)# ip ospf network point-to-multipoint non-broadcast
R3(config-if)# no shutdown
R3(config-if)# 
R3(config-if)# interface GigabitEthernet0/4
R3(config-if)# ip address 10.0.34.3 255.255.255.0
R3(config-if)# ip ospf network non-broadcast
R3(config-if)# no shutdown
R3(config-if)# 
R3(config-if)# router ospf 1
R3(config-router)# router-id 3.3.3.3
R3(config-router)# neighbor 10.0.23.2
R3(config-router)# neighbor 10.0.34.4
R3(config-router)# network 10.0.23.3 0.0.0.0 area 0
R3(config-router)# network 10.0.34.3 0.0.0.0 area 0

Router R4

Router(config)# hostname R4
R4(config)# 
R4(config)# interface GigabitEthernet0/1
R4(config-if)# ip address 10.0.14.4 255.255.255.0
R4(config-if)# ip ospf network point-to-multipoint
R4(config-if)# no shutdown
R4(config-if)# 
R4(config-if)# interface GigabitEthernet0/2
R4(config-if)# ip address 10.0.24.4 255.255.255.0
R4(config-if)# ip ospf network broadcast
R4(config-if)# no shutdown
R4(config-if)# 
R4(config-if)# interface GigabitEthernet0/3
R4(config-if)# ip address 10.0.34.4 255.255.255.0
R4(config-if)# ip ospf network non-broadcast
R4(config-if)# no shutdown
R4(config-if)# 
R4(config-if)# router ospf 1
R4(config-router)# router-id 4.4.4.4
R4(config-router)# neighbor 10.0.34.3
R4(config-router)# network 10.0.14.4 0.0.0.0 area 0
R4(config-router)# network 10.0.24.4 0.0.0.0 area 0
R4(config-router)# network 10.0.34.4 0.0.0.0 area 0

Examples 1, 2, 3, 4, and 5 show the default Hello and Dead interval values for all OSPF network types.

R1# show ip ospf interface gigabitEthernet 0/2
GigabitEthernet0/2 is up, line protocol is up 
  Internet Address 10.0.12.1/24, Area 0, Attached via Network Statement
  Process ID 1, Router ID 1.1.1.1, Network Type POINT_TO_POINT, Cost: 1
  Topology-MTID    Cost    Disabled    Shutdown      Topology Name
        0           1         no          no            Base
  Transmit Delay is 1 sec, State POINT_TO_POINT
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5


omitted output

Example 1 – OSPF timers for a point-to-point OSPF-enabled interface

R1# show ip ospf interface gigabitEthernet 0/4
GigabitEthernet0/4 is up, line protocol is up 
  Internet Address 10.0.14.1/24, Area 0, Attached via Network Statement
  Process ID 1, Router ID 1.1.1.1, Network Type POINT_TO_MULTIPOINT, Cost: 1
  Topology-MTID    Cost    Disabled    Shutdown      Topology Name
        0           1         no          no            Base
  Transmit Delay is 1 sec, State POINT_TO_MULTIPOINT
  Timer intervals configured, Hello 30, Dead 120, Wait 120, Retransmit 5


omitted output

Example 2 – OSPF timers for a point-to-multipoint OSPF-enabled interface

R2# show ip ospf interface gigabitEthernet 0/3
GigabitEthernet0/3 is up, line protocol is up 
  Internet Address 10.0.23.2/24, Area 0, Attached via Network Statement
  Process ID 1, Router ID 2.2.2.2, Network Type POINT_TO_MULTIPOINT, Cost: 1
  Topology-MTID    Cost    Disabled    Shutdown      Topology Name
        0           1         no          no            Base
  Transmit Delay is 1 sec, State POINT_TO_MULTIPOINT
  Timer intervals configured, Hello 30, Dead 120, Wait 120, Retransmit 5


omitted output



R2# show running-config interface  gigabitEthernet 0/3
Building configuration...

Current configuration : 165 bytes
!
interface GigabitEthernet0/3
 ip address 10.0.23.2 255.255.255.0
 ip ospf network point-to-multipoint non-broadcast
 duplex auto
 speed auto
 media-type rj45
end

Example 3 – OSPF timers for a non-broadcast point-to-multipoint OSPF-enabled interface

R2# show ip ospf interface gigabitEthernet 0/4
GigabitEthernet0/4 is up, line protocol is up 
  Internet Address 10.0.24.2/24, Area 0, Attached via Network Statement
  Process ID 1, Router ID 2.2.2.2, Network Type BROADCAST, Cost: 1
  Topology-MTID    Cost    Disabled    Shutdown      Topology Name
        0           1         no          no            Base
  Transmit Delay is 1 sec, State DR, Priority 1
  Designated Router (ID) 2.2.2.2, Interface address 10.0.24.2
  Backup Designated router (ID) 4.4.4.4, Interface address 10.0.24.4
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5


omitted output

Example 4 – OSPF timers for a broadcast OSPF-enabled interface

R3# show ip ospf interface gigabitEthernet 0/4
GigabitEthernet0/4 is up, line protocol is up 
  Internet Address 10.0.34.3/24, Area 0, Attached via Network Statement
  Process ID 1, Router ID 3.3.3.3, Network Type NON_BROADCAST, Cost: 1
  Topology-MTID    Cost    Disabled    Shutdown      Topology Name
        0           1         no          no            Base
  Transmit Delay is 1 sec, State BDR, Priority 1
  Designated Router (ID) 4.4.4.4, Interface address 10.0.34.4
  Backup Designated router (ID) 3.3.3.3, Interface address 10.0.34.3
  Timer intervals configured, Hello 30, Dead 120, Wait 120, Retransmit 5


omitted output

Example 5 – OSPF timers for a non-broadcast OSPF-enabled interface

Why Adjusting the Hello and Dead Intervals?

A Higher Hello interval causes OSPF adjacencies to take a long time to get built up. Therefore, lowering the Hello timer’s interval speeds up the process of forming full OSPF neighbor relationships. However, that would generate more OSPF routing traffic.

To sum up, the shorter the hello interval, the faster network topology changes will be noticed.

On the other hand, the higher the Dead interval, the slower the OSPF convergence after a failure. On a non-broadcast network, the dead timer is 120 seconds. If an OSPF node’s interface, connected to a non-broadcast network segment, fails, all its neighbors on that segment will wait 120 seconds before considering the router down and updating their LSDBs based on this change. To circumvent this issue, we need to decrease the Dead interval.

Finally, it is likely to improve convergence if a lower dead interval is defined. However, it may result in additional routing fluctuation because the likelihood of not receiving hellos within a short period of time increases.

Configuring the Hello and Dead Intervals

Configuring The Hello Interval

The OSPF Hello interval is the time period between two hello packets that the Cisco IOS software sends on the interface, and is announced in the hello packets.

To set the Hello interval’s value for a particular interface, use the ip ospf hello-interval scnds command in interface configuration mode, where scnds defines the interval in seconds.

The value ranges from 1 to 65535. In a particular network segment, the Hello interval should be the same on all OSPF-enabled interfaces connected to that network for the routers to build up OSPF adjacencies.

The following example configures the Hello interval in subnet 10.0.14.0/24 to 5 seconds:
Router R1

R1(config)# interface gigabitEthernet 0/4
R1(config-if)# ip ospf hello-interval 5

Router R4

R4(config)# interface gigabitEthernet 0/1
R4(config-if)# ip ospf hello-interval 5

The verify our configuration, we use the show ip ospf interface command, as you see in Example 6.

R1# show ip ospf interface gigabitEthernet 0/4
GigabitEthernet0/4 is up, line protocol is up 
  Internet Address 10.0.14.1/24, Area 0, Attached via Network Statement
  Process ID 1, Router ID 1.1.1.1, Network Type POINT_TO_MULTIPOINT, Cost: 1
  Topology-MTID    Cost    Disabled    Shutdown      Topology Name
        0           1         no          no            Base
  Transmit Delay is 1 sec, State POINT_TO_MULTIPOINT
  Timer intervals configured, Hello 5, Dead 20, Wait 20, Retransmit 5


omitted output

Example 6 – OSPF timers on R1’s GigabitEthernet 0/4 interface

Note that the Dead interval is four times the Hello interval configured by the ip ospf hello-interval command. Therefore, you can adjust the Dead interval by adjusting the Hello time period. Finally, to restore the default Hello interval, use the no ip ospf hello-interval command.

Configuring The Dead Interval

The OSPF Dead interval is announced in OSPF hello packets. It is the amount of time during which the router waits to receive at least one hello packet from a neighbor before considering that neighbor down.

To configure the Dead interval for a particular interface, use the ip ospf dead-interval scnds command in interface configuration mode, where scnds is a time period in seconds that ranges from 1 to 65535.

Like the Hello interval, the Dead interval value should be the same for all OSPF-enabled interfaces connected to the current network segment. In this example, we set the Dead interval in subnet 10.0.23.0/24 to 60 seconds.

Router R2

R2(config)# interface gigabitEthernet 0/3
R2(config-if)# ip ospf dead-interval 60

Router R3

R3(config)# interface gigabitEthernet 0/2
R3(config-if)# ip ospf dead-interval 60

To verify our configuration, we use the show ip ospf interface command in enable mode (Example 7).

R3# show ip ospf interface gigabitEthernet 0/2
GigabitEthernet0/2 is up, line protocol is up 
  Internet Address 10.0.23.3/24, Area 0, Attached via Network Statement
  Process ID 1, Router ID 3.3.3.3, Network Type POINT_TO_MULTIPOINT, Cost: 1
  Topology-MTID    Cost    Disabled    Shutdown      Topology Name
        0           1         no          no            Base
  Transmit Delay is 1 sec, State POINT_TO_MULTIPOINT
  Timer intervals configured, Hello 30, Dead 60, Wait 60, Retransmit 5


omitted output

Example 7 – OSPF timers on R3’s GigabitEthernet 0/2 interface
Finally, to restore the default Dead interval on a specific interface, use the no ip ospf dead-interval command without indicating the current configured value.

Configuring OSPF Fast Hello Packets

OSPF fast hello packets are normal OSPF Hello packets, which are sent many times per second with a dead interval of 1 second. The hello interval advertised in fast hello packets is set to 0, as you see in Figure 4.

Figure 4 – Fast OSPF hello sent by R4 to R2

One disadvantage of the OSPF fast hello feature is that there is no guarantee that the hello packets will be sent at the rate you have configured if the router’s CPU is overloaded.

To enable fast hellos, use the ip ospf dead-interval minimal hello-multiplier mltplr, where mltplr is the number of hello packets to be sent every second. The value is an integer that ranges from 3 to 20. Additionally, the command sets the dead interval to 1 second.

The following example enables OSPF fast hello packets on subnet 10.0.24.0/24, sets the dead interval to 1 second, and instructs R2 and R4 to generate 10 hello packets every second (every 100ms).

Router R2

R2(config)# interface gigabitEthernet 0/4
R2(config-if)# ip ospf dead-interval minimal hello-multiplier 10

Router R4

R4(config)# interface gigabitEthernet 0/2
R4(config-if)# ip ospf dead-interval minimal hello-multiplier 10

To verify the dead interval and fast hello interval, we use the show ip ospf interface command (Example 8).

R2# show ip ospf interface gigabitEthernet 0/4
GigabitEthernet0/4 is up, line protocol is up 
  Internet Address 10.0.24.2/24, Area 0, Attached via Network Statement
  Process ID 1, Router ID 2.2.2.2, Network Type BROADCAST, Cost: 1
  Topology-MTID    Cost    Disabled    Shutdown      Topology Name
        0           1         no          no            Base
  Transmit Delay is 1 sec, State BDR, Priority 1
  Designated Router (ID) 4.4.4.4, Interface address 10.0.24.4
  Backup Designated router (ID) 2.2.2.2, Interface address 10.0.24.2
  Timer intervals configured, Hello 100 msec, Dead 1, Wait 1, Retransmit 5



omitted output

Example 8 – OSPF timers on R2’s GigabitEthernet 0/4 interface

When you activate fast OSPF hellos on a particular network segment, the hello multiplier may not be the same on all router interfaces because OSPF nodes ignore the hello interval values embedded in the hello packets exchanged in that segment. However, the dead interval must be consistent on the segment.

In this example, we set the hello-multiplier for R2’s G0/4 interface.

R2(config)# interface gigabitEthernet 0/4
R2(config-if)# ip ospf dead-interval minimal hello-multiplier 5

As shown in Example 9, router R2 still forms full OSPF neighbor relationship with router R4 even the hello-multiplier values configured on the interfaces connecting them are different.

R2# show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
4.4.4.4           1   FULL/DR         923 msec    10.0.24.4       GigabitEthernet0/4
3.3.3.3           0   FULL/  -        00:00:49    10.0.23.3       GigabitEthernet0/3
1.1.1.1           0   FULL/  -        00:00:39    10.0.12.1       GigabitEthernet0/1

Example 9 – Displaying R2’s OSPF neighbors

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Mohamed Ouamer is a computer science teacher and a self-published author. He taught networking technologies and programming for more than fifteen years. While he loves to share knowledge and write, Mohamed's best passions include spending time with his family, visiting his parents, and learning new things.