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which path will traffic from R6 take to reach R1?

Referring to the OSPF link metrics in the exhibit, which path will traffic from R6 take to reach R1?

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A.
R6, R3, R2, R4, R1

B.
R6, R3, R2, R1

C.
R6, R5, R4, R1

D.
R6, R5, R3, R2, R4, R1

10 Comments on “which path will traffic from R6 take to reach R1?

  1. Nguyen Duc Hoa says:

    B is correct.
    When R1 advertises a subnet intto Area 1, R2 and R4 will receive it. R4 will generate a LSA 3 and advertise into Area 0. So, from R6 to R2, R2 will have two path to reach a subnet in Area 1. One is O and one is OIA, and O is prefered (although its cost is higher).




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  2. jwmcinsc says:

    rawmaterial has a good point. I was sold on Ducs explanation until I read raws question. Anyone care to detail why the same decision that influenced the path to use R2 according to the answer being B as opposed to R4? As raw says, wouldn’t they both generate an LSA 3? I don’t think enough information is provided here.




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  3. GSU74 says:

    My understanding based on Moy’s OSPF Anatomy is that that both R2 and R4 would send a Type 3 (Summary) LSA into area 0 with the same cost which is the cost to the most distant component = 25. The LSA 3 is sent to R3 and R5. R3 creates a new LSA 3 for the path thru R2 = 25 + 5 = 30, and sends it to R6. R5 creates a new LSA 3 for the path thru R3 & R2 = 25 + 10 + 5 = 40 and also send it to R6. R6 will pick the R6-R3-R2-R1 path (B).




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  4. Dylan says:

    I am trying to follow Nguyen Duc Hoa’s explanation and broke it down to 5 steps below.
    I don’t understand how he came to determine Answer B is correct on point (5) below.

    (1) B is correct.
    (2) When R1 advertises a subnet intto Area 1, R2 and R4 will receive it.
    (3) R4 will generate a LSA 3 and advertise into Area 0.
    (4) So, from R6 to R2, R2 will have two path to reach a subnet in Area 1.
    (5) One is O (intra-area) and one is OIA (inter-area), and O (intra-area) is prefered (although its cost is higher).

    Since R2 has two paths to R1, one directly through R1 (OIA – inter-area) and other one through R4 (O – intra-area)
    And we know that O – Intra area path is prefered, then R2 will prefer the path through R4.
    Which makes answer A correct.




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  5. Dylan says:

    I believe B is the answer
    Please see RFC 2328 below and how it defines intra-area path.

    If intra area paths are destinations belonging to one of the routers attached areas, then since R2 is attached to area 0 and area 1, then the path from R2 to R1 is an intra-area path. And the route from R1 advertised through R4 is coming in as LSA 3 (summary LSA) hence becomes an inter-area path.
    This is how answer B becomes correct.

    RFC 2328 section 11

    http://www.ietf.org/rfc/rfc2328.txt
    Path-type
    There are four possible types of paths used to route traffic to
    the destination, listed here in decreasing order of preference:
    intra-area, inter-area, type 1 external or type 2 external.
    Intra-area paths indicate destinations belonging to one of the
    router’s attached areas. Inter-area paths are paths to
    destinations in other OSPF areas. These are discovered through
    the examination of received summary-LSAs. AS external paths are
    paths to destinations external to the AS. These are detected
    through the examination of received AS-external-LSAs.

    also see explanation in this link
    https://supportforums.cisco.com/thread/2239499




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