07-26-2016 06:53 AM - edited 03-07-2019 12:16 AM
From R1's perspective what is best path for external prefix 5.5.5.5/32..
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07-26-2016 08:57 AM
Hi,
R1 has little choice - it has to go through R2. However, R2 does have a choice, and I personally believe that R2 will use both paths through R3 and R4, assuming the link costs are equal.
R2 will see a single LSA5 originated by R5 and flooded through Area 0. While R5 will also originate an LSA7 into the NSSA area, this LSA will have the P-bit cleared as required by RFC 3101 (RFC 1587), and so will not be translated into LSA5 by R3. We should therefore focus on the way R2 will handle this single LSA5.
R2 will see two LSA5, one received through Area 0 originated by R5, the other originated by R3 as a translation of LSA7 into LSA5. The LSA5 from R5 has its Forwarding Address set to 0.0.0.0; the LSA5 from R3 has the forwarding address set to R5.
On R2, the process will be:
So, in effect, if this analysis is correct, you are using both paths to reach 5.5.5.5/32, assuming that the link costs are equal.
Would this align with your observation if you configured this lab?
Best regards,
Peter
EDIT: Corrected a misleading explanation that assumed that LSA7 originated by R5 will be translated on R3 - it won't because R5 is an ABR and originates a LSA5 for 5.5.5.5/32 into the backbone already, so there is no point in having the same LSA7 translated into another LSA5 by R3. That's why the P-bit in LSA7 will be cleared, preventing it from being translated into LSA5.
07-26-2016 08:24 AM
What are the costs of the links? There is no information regarding that in the diagram.
Assuming equal cost links there will be equal cost load balancing on paths R1-R2-R4-R5 and R1-R2-R3-R5.
07-26-2016 08:35 AM
hello dhananjay95929,,,In above topology,R1-R2cost 1,In area 0 that belong R2,R3 and R4 are cost 1.R4-R5 cost 64,and R3-R5 cost 1
07-26-2016 08:57 AM
Hi,
R1 has little choice - it has to go through R2. However, R2 does have a choice, and I personally believe that R2 will use both paths through R3 and R4, assuming the link costs are equal.
R2 will see a single LSA5 originated by R5 and flooded through Area 0. While R5 will also originate an LSA7 into the NSSA area, this LSA will have the P-bit cleared as required by RFC 3101 (RFC 1587), and so will not be translated into LSA5 by R3. We should therefore focus on the way R2 will handle this single LSA5.
R2 will see two LSA5, one received through Area 0 originated by R5, the other originated by R3 as a translation of LSA7 into LSA5. The LSA5 from R5 has its Forwarding Address set to 0.0.0.0; the LSA5 from R3 has the forwarding address set to R5.
On R2, the process will be:
So, in effect, if this analysis is correct, you are using both paths to reach 5.5.5.5/32, assuming that the link costs are equal.
Would this align with your observation if you configured this lab?
Best regards,
Peter
EDIT: Corrected a misleading explanation that assumed that LSA7 originated by R5 will be translated on R3 - it won't because R5 is an ABR and originates a LSA5 for 5.5.5.5/32 into the backbone already, so there is no point in having the same LSA7 translated into another LSA5 by R3. That's why the P-bit in LSA7 will be cleared, preventing it from being translated into LSA5.
07-26-2016 08:57 AM
hello peter, oops pressed "corrected option"instead "reply" can't see how to take correct ans off now...
'....
but i have one doubt.. suppose 5.5.5.5/32 are redistribute via eigrp.when nssa border router originate both type -5 and type-7 lsa for same network p-bit must be cleared in the type-7 lsa so R3 don't translate packet type...please correct me if i am wrong...
07-26-2016 08:57 AM
Hello,
You're perfectly right... I totally missed that! Yes, you are correct! If an NSSA ASBR originates both LSA7 and LSA5 for a network, the LSA7 will have the P-bit cleared, so it will not be translated into LSA5 by another ABR. So in my previous explanation, the whole part about the translated LSA5 does not hold because there will be no such LSA at all.
I will correct my explanation above.
Please keep in mind that my previous explanation holds only if the link costs are the same. If they are not (and you have later said that they are different indeed), the paths to R5 may end up being different.
Best regards,
Peter
07-26-2016 09:09 AM
hello peter..so In my topology path will be R1-R2-R4-R5..Am i right..
But i am not able to understand what is purpose ospf forward address....And why we enable ospf into no-ospf domain for non-zero forward address...can u explain detail
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