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Default route redistributed from NSSA ABR/ASBR (area 4) getting withdrawn on NSSA ASBR (area 4) when turning up new link on a different area.

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  • 1.  Default route redistributed from NSSA ABR/ASBR (area 4) getting withdrawn on NSSA ASBR (area 4) when turning up new link on a different area.

    Posted 06-27-2020 22:38

    Hello Experts,

     

    Below is a simple topology with R1 participating in Area 0 and Area 4(configured for NSSA) and R2 participating in Area 4.

    R1 being the NSSA ABR/ASBR is redistributing a default route and a bunch of other static routes configured locally using the 'ospf export' command.

     

      static route redistribution

                   |

                   |

    (Area0) R1 (Area4) ---- (Area4) R2

     

    Per  RFC 3101 since R1 is also connected to another area and is redistributing a route from another protocol (static) it is clearing the P-bit in the Type-7 LSA's sent towards R2 to avoid further propagation to its connected areas.

     

    All works fine and we are observing a type-7 default route alongwith the redistributed static routes in the ospf database and subsequently in the routing table.

     

    Now when i enable any random interface on R2 to participate in any other area apart from area 4, i lose only the default route in the routing table but not the other static routes from the same type-7 LSA's.

     

    Upon looking in the database on R2, i can find the type-7 default route along with the other type-7 routes redistributed from R1, with the same values in the options field, but only the default route is withdrawn from the routing table.

     

    Upon de-activation of the recent change, the default route gets re-installed.

     

    This behaviour is seen in the customer topology with R1 being a MX960 and R2 being a QFX5100.

    The same was replicated in the LAB using R1 & R2 as EX3300.

     

    Is this an expeteced behaviour of withdrawing only the default route, but keeping all other Type-7 routes, from the routing table when a NSSA ASBR is connected to multiple areas?

     

    Any help is appreciated. Thank you in advance !

     

    *Note: Configuration files & ospf database outputs uploaded and named accordingly.

     

     



  • 2.  RE: Default route redistributed from NSSA ABR/ASBR (area 4) getting withdrawn on NSSA ASBR (area 4) when turning up new link on a different area.

    Posted 06-27-2020 23:40

    Hello Experts,

     

    Due to some reason i am unable to upload the files here, instead sharing a dropbox link with all the files uploaded.

     

    https://www.dropbox.com/sh/ghylulqyp34vn8s/AACO2LjZhBf8Hn82U5_L5gLAa?dl=0

     

    Regards,

    Prathmesh D



  • 3.  RE: Default route redistributed from NSSA ABR/ASBR (area 4) getting withdrawn on NSSA ASBR (area 4) when turning up new link on a different area.
    Best Answer

    Posted 06-28-2020 00:03

    Hello,

     


    @prathmeshvid wrote:

     

    Now when i enable any random interface on R2 to participate in any other area apart from area 4, i lose only the default route in the routing table but not the other static routes from the same type-7 LSA's.

     

    <skip>

     

    Upon de-activation of the recent change, the default route gets re-installed.

     

    <skip>

     

    Is this an expeteced behaviour of withdrawing only the default route, but keeping all other Type-7 routes, from the routing table when a NSSA ASBR is connected to multiple areas?

     


     

    This is expected. From RFC 3101 section 2.5  https://tools.ietf.org/html/rfc3101#section-2.5 :

     

    if the destination is a Type-7 default route (destination
              ID = DefaultDestination) and one of the following is true,
              then do nothing with this LSA and consider the next in the
              list:
                o  The calculating router is a border router and the LSA has
                   its P-bit clear. 

     

     

    When You "turn on" a link + add an extra area on R4, You make R4 an NSSA Area Border Router/ABR, and 0/0 with P-bit clear does not get installed on R4.

     

    HTH

    Thx

    Alex

     

     

    P.S. Welcome to just one of many unforessen consequences of OSPF NSSA, BTW.

    That's why I never use OSPF NSSA in my designs despite advocates of OSPF NSSA preaching how wonderful it is because of <whatever> Smiley LOL

     

     

     

     



  • 4.  RE: Default route redistributed from NSSA ABR/ASBR (area 4) getting withdrawn on NSSA ASBR (area 4) when turning up new link on a different area.

    Posted 06-28-2020 02:12

    Thank you so much @aarseniev. This documentation was exactly what i was looking for. I was scratching my head over this particular snippet for over 3 days, might have just overlooked this.

     

    Thanks anyways for your prompt help. Have a great day !!!