OSPF Hello and Dead Interval

This topic is to discuss the following lesson:

If you change the hello and dead time on R1, do you have to make the exact same change on the R2 interface? Do the hello and dead timers have to be the same on both routers?

Hi Jason,

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

Rene

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

Onur,
Using Bi-Directional Forwarding Detection is generally considered to be the better option for a couple of reasons:

  1. BFD can detect a downed neighbor much faster than any OSPF timer manipulation
  2. BFD is less CPU intensive than very small OSPF timers because BFD is pushed off to the data plane, whereas OSPF timer events must be processed by the router’s control plane

Dear Rene,

Many Thanks to you .One question…

Why Neighbor will not form if Hello/dead Mismatch ?? Could you please explain behind the reason on it ? I think if Hello/Dead is not same on both end then there could be operational related problem like One router release neighbor first while other router still wait, right ??

br//
zaman

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, Hello Packetsare also used to elect the Designated Router and BackupDesignated Router.The format of an Hello packet is detailed in Section A.3.2. The Hello packet contains the router’s Router Priority (used in choosing the Designated Router), and the interval between Hello Packets sent out the interface (HelloInterval). The Hello Packet also indicates how often a neighbor must be heard from to remain active (RouterDeadInterval). Both HelloInterval and
RouterDeadInterval must be the same for all routers attached to a common network. The Hello packet also contains the I
P address mask of the attached network (Network Mask). On unnumbered point-to-point networks and on virtual links this field should be set to 0.0.0.0.”

In other word to say, this is how OSPF works as per Hello & Dead interval to form a neighbor-ship with another router… Additionally, to form neighbor-ship with another OSPF router, there is also other fields to be matched in addition of the hello/deal interval which are:
- Area id
- Authentication password
- Stub area Flag

All those are contained in the Hello packet sent from neighbor router(s) they must be matched. I really advise you to check this lesson where you can find more information about this topic:
Introduction of OSPF

Hope I could answer your question.

Hi Rene,

I have two established neighbors, just take dead interval as 40 secs if dr goes down for 30 secs bdr will wait for dead interval to proceed further or will it become as dr ??

Hi Karthik,

If your dead interval is 40 seconds then OSPF will wait for 40 seconds until it declares the neighbor as dead, deleting the neighbor adjacency. If you want to see this in action, connect two routers to each other with a switch in the middle, then disconnect one of the links :slight_smile:

Rene

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

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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,

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