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Forum Replies

  1. 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?

  2. Hi Jason,

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


  3. 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
  4. 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 ??


  5. maher1 says:

    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”

    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.

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