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  • 1.  Self-generated LSA aging on flood reduction enabled router

    Posted 02-17-2024 13:18
    Edited by LEEBAHI 02-17-2024 13:19

    Hi every one,

    Please consider the following set up:

    R1 OSPD ID:

    R6 OSPF ID:

    R1 is creating  LSA1 with DNA bit set because gig-0/0/0 is configured with " flood reduction"

    R1 sends the LSA1 with DNA bit set to R6 which simply stores in OSPF database  and will not age the LSA1 from R1 as expected. From that point on, this  LSA1 will be replaced if R1 sees  a change , only then it will send LSA1 with the change.

    However, R1 will continue to age its own self-generated LSA1 and once it expires, it will generate a new LSA1 with a new seq number, But R1 will not send it to R6, as there is no change . This will result R6 having old LSA1 with old seq # in its ospf database while R1 has newer LSA1 with higher sequence number.

    After a few seconds, we ran the command again and see age does not change  as expected.

    R1 however will continue to age this self-generated LSA :

    Note lack of * under age column.

    After a few seconds, we ran the command again and see  LSA1 is ageing:

    Eventually R1 will age out this LSA1 and a new LSA1 will be generated ,but will not be flooded to R6 so R6 will have older LSA with DNA bit set.


    This behavior is based on RFC 4136:

    Flooding-reduction-capable routers will continue to send hellos to
       their neighbors and keep aging their self-originated LSAs in their
       database.  However, these routers will flood their self-originated
       LSAs with the DoNotAge bit set

    What is the point of aging self-generated LSA on router configured with flood reduction such as R1 in our example and regenerating them when they expire?

    Much appreciated!!

    Be kind!!

  • 2.  RE: Self-generated LSA aging on flood reduction enabled router

    Posted 02-19-2024 08:01


    I just wanna note that router gets LSAs with DNA bit and tick them do not countdown time in the database. 
    At the same time router does not get any LSAs with DNA bit, so it goes on to operate with LSAs in database in normal way.

    "all the LSAs generated by the routes that traverse the specified interface have the DoNotAge bit set
    when they are initially flooded, and LSAs are refreshed only when a change occurs.



    Dmitry Maksimov