Routing

Hi Experts,

 

I was reading up on OSPF and thought of this question:

 

 

R1                         R2

 |                            |

  \                          /       =============> area 1

    \                      /

      \                  /

             R3

 

-- R1 and R2 are ABR's (connected to area 0)

-- area-1 is an NSSA

-- R3 is the ASBR redistributing a route into OSPF

 

Now, I understand that amongst R1 and R2, the router with higher RID will do the type-7 to type-5 translation.  However the question is:  R1 and R2 are not neighbors. R3 will flood a type-7 LSA into area-1. This means that both R1 and R2 receive it, but since R1 and R2 are not neighbors, how do they decide which of them will do the translation? Does it mean that initally both of them do the translation and when they receive each others' LSA in area-0, the one with inferior RID pulls its LSA back?

 

Started thinking about this and now can't get past it..Please let me know..

 

cheers,

ankit

 

 

Hi

 

I would think this way: R1 and R2 are both connected to Area 1 so they have full LSDB of Area 1. In particular, they are aware of all other routers (RIDs) in Area 1 and which ones are ABRs. So R1 or R2 will know if they are ABR with higest RID or not. So each router will either do translation or not. 

great.. just took a closer look at the intra-area LSA. The 'B' bit is set..
so they know who all are the ABRs and correct decision can be taken.
Many thanks for your reply!

cheers,
ankit

Hi Ankit,

Inside the NSSA area, Router-LSAs (Type-1) will be flooded.
This Type-1 LSAs have area-flooding scope and all routers in that area will be seeing this LSAs.
 
 
This Router LSAs have many fields. ( Eg-  Link ID, Link Type, Link Data, Metric etc).

Also, it will have 1 octet V/E/B bits field which describes some characters of the router which generated that LSA.


( Text from JNCIP study guide - http://www.juniper.net/us/en/training/certification/JNCIS_studyguide.pdf )

V/E/B Bits (1 octet)
This field contains five leading zeros followed by the V, E, and B bits.

These bits convey the characteristics of the local router. The various bit definitions are:

V bit - The V bit is set when the local router is an endpoint for one or more fully operational virtual links.

E bit  - The E bit is set when the local router is configured as an AS boundary router (ASBR) to inject external routes into the network.

B bit - The B bit is set when the local router has configured interfaces in more than one OSPF area, thereby turning the router into an ABR.


Therefore, all routers in the NSSA (including the ABRs) will know who are the ABRs and their Router-Ids . ( by recieving the LSAs with B bit)

If an ABR finds itself as having the highest RID, it will start doing the translation.

Hope this helps.

 

Regards

Moses N

 

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@Moses: yeps, got it.
thanks!