OSPF Hello and Dead Interval

OSPF uses hello packets and two timers to check if a neighbor is still alive or not:

  • Hello interval: this defines how often we send the hello packet.
  • Dead interval:  this defines how long we should wait for hello packets before we declare the neighbor dead.

The hello and dead interval values can be different depending on the OSPF network type. On Ethernet interfaces you will see a 10 second hello interval and a 40 second dead interval.

Let’s take a look at an example so we can see this in action. Here’s the topology I will use:

OSPF R1 SW R2

We’ll use two routers with a switch in between.

Configuration

Let’s enable OSPF:

R1 & R2#
(config)#router ospf 1
(config-router)#network 192.168.12.0 0.0.0.255 area 0

Let’s take a look at the default hello and dead interval:

R1#show ip ospf interface FastEthernet 0/0 | include intervals
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5

The hello and dead interval can be different for each interface. Above you can see that the hello interval is 10 seconds and the dead interval is 40 seconds. Let’s try if this is true:

R1(config)#interface FastEthernet 0/0
R1(config-if)#shutdown 

After shutting the interface on R1 you will see the following message:

R1#
Aug 30 17:57:05.519: %OSPF-5-ADJCHG: Process 1, Nbr 192.168.12.2 on FastEthernet0/0 from FULL to DOWN, Neighbor Down: Interface down or detached

R1 will know that R2 is unreachable since its interface went down. Now take a look at R2:

R2#
Aug 30 17:57:40.863: %OSPF-5-ADJCHG: Process 1, Nbr 192.168.12.1 on FastEthernet0/0 from FULL to DOWN, Neighbor Down: Dead timer expired

R2 is telling us that the dead timer has expired. This took a bit longer. The interface on R1 went down at 17:57:05 and R2’s dead timer expired at 17:57:40…that’s close to 40 seconds.

Let’s activate the interface again:

R1(config)#interface FastEthernet 0/0
R1(config-if)#no shutdown

40 seconds is a long time…R2 will keep sending traffic to R1 while the dead interval is expiring. To speed up this process we can play with the timers. Here’s an example:

R1 & R2
(config)#interface FastEthernet 0/0
(config-if)#ip ospf hello-interval 1 
(config-if)#ip ospf dead-interval 3

You can use these two commands to change the hello and dead interval. We’ll send a hello packet every second and the dead interval is 3 seconds. Let’s verify this:

R1#show ip ospf interface FastEthernet 0/0 | include intervals
  Timer intervals configured, Hello 1, Dead 3, Wait 3, Retransmit 5

Reducing the dead interval from 40 to 3 seconds is a big improvement but we can do even better:

We're Sorry, Full Content Access is for Members Only...

If you like to keep on reading, Become a Member Now! Here is why:

  • Learn any CCNA, CCNP and CCIE R&S Topic. Explained As Simple As Possible.
  • Try for Just $1. The Best Dollar You've Ever Spent on Your Cisco Career!
  • Full Access to our 657 Lessons. More Lessons Added Every Week!
  • Content created by Rene Molenaar (CCIE #41726)

528 Sign Ups in the last 30 days

satisfaction-guaranteed
100% Satisfaction Guaranteed!
You may cancel your monthly membership at any time.
No Questions Asked!

Forum Replies

  1. Hi Jason,

    These have to be match yes, otherwise no neighbor adjacency is established.

    Rene

  2. Dear Rene,
    Could enabling BFD be better option than reducing dead-hello timers regarding the consuming router resource?
    Which method do you recommend ?

  3. Hi Mohammad,

    A good question. I am referring to you to RFC which is a type of publication from the Internet Engineering Task Force (IETF) and the Internet Society (ISOC), the principal technical development and standards-setting bodies for the Internet.
    On April 1998, they have created the publication RFC 2328 for OSPF version 2, and on point 9.5 they spoke about “Sending Hello Packets” as following:

    “Hello packets are sent out each functioning router interface. They are used to discover and maintain neighbor relationships.[6] On broadcast and NBMA networks, He

    ... Continue reading in our forum

  4. sir
    1> you have this command ip ospf dead-interval minimal hello-multiplier 3 why you have used 3 what is the meaning of minimal hello-multiplier 3 , I understand that when we select minimal the =n our dead interval time will be 1 but why you used hello-multiplier 3 please explain it but if i am using 4 instead of 3 my neighbor adjancecy is working on R1 i have used hello-multiplier 3 and R2 i have used hello-multiplier 4 and they have neighbor adjancency please explain it

  5. Hello Bhai,

    The “3” in that command means how many hello packets will be sent in 1 second period. Also when you use that command, that is ip ospf dead-interval minimal hello-multiplier x; The x number or the number of hello packets sent per second doesn’t need to match between the two neighbors.

    The main important thing about this feature is for critical environment to be able to detect the loss of neighborship in 1 second.

    Regards,

5 more replies! Ask a question or join the discussion by visiting our Community Forum