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OSPF link metrics and The Path

Level 1
Level 1

     Experts ,

Why answer is B ? Why route chosen is not  R6, R3, R2, R4, R1 when it has least cost ?

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

A. R6, R3, R2, R4, R1

B. R6, R3, R2, R1

C. R6, R5, R4, R1

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

Answer: B

3 Replies 3

Level 1
Level 1

Because R2 is the ABR that connects Area 0 to Area 1, therefore it has databases for both areas and OSPF always prefers Intra-Area paths rather than Inter-Area Paths.

Jose is correct, IOS still uses the path selection as defined in RFC 1583 (for backwards-compatibility):


There are four possible types of paths used to route traffic to

the destination, listed here in 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 link advertisements.  AS external paths are

paths to destinations external to the AS.  These are detected

through the examination of received AS external link


This has been changed in RFC 2328, which is the default in NX-OS:

Intra-area paths using non-backbone areas are always the most preferred

The other paths, intra-area backbone paths and inter-area paths, are of equal preference

You can change the behavior with the [no] compatible rfc... command.

The order of preference takes priority over path cost and even administrative distance.

If the link between R2 and R4 would belong to area 1, the least cost path (via R4) would be prefered.

Hope that helps


P.S.: What exam is that question from?

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