Routing

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

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(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the other type-7 routes redistributed from R1, with the same values in the options field, but only the default route is withdrawn from the routing table.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.

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

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?

 

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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> 

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 !!!