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Re: OSPF path selection posted 01/03/2007
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Actually it is not that straightforward. RFC says intra area route is
preferred when intra-area path is using non-backbone area, what happens if
the intra-area is backbone area?

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

           o   The other paths, intra-area backbone paths and inter-
               area paths, are of equal preference.
End Quote

Jian

On 1/2/07, Gavin Lawson <GavinL@xxxxxxxxxxxx> wrote:
>
> OSPF will select routes in the following order.
> . Intra area route.
> . Inter area route.
> . External route.
>
> The cost is look at after the area identifier.
> So if an external route has a lower cost than an intra area route - the
> intra area route will still be selected.
>
> GL
>
> -----Original Message-----
> From: nobody@xxxxxxxxxxxxxx [mailto:nobody@xxxxxxxxxxxxxx] On Behalf Of
> Jian Gu
> Sent: Wednesday, 3 January 2007 11:33 AM
> To: Cisco certification
> Subject: OSPF path selection
>
> Hi, all,
>
> If a router receives an external route with type 1 metric coming from
> intra-area (higher cost) and inter area lower cost from the same
> originator, how OSPF selects the routes? I am having hard time to grasp
> how RFC says in this scenario.
>
> ********* RFC2328 ***************
>
> 16.4.1.  External path preferences
>
>             When multiple intra-AS paths are available to
>             ASBRs/forwarding addresses, the following rules indicate
>             which paths are preferred. These rules apply when the same
>             ASBR is reachable through multiple areas, or when trying to
>             decide which of several AS-external-LSAs should be
>             preferred. In the former case the paths all terminate at the
>             same ASBR, while in the latter the paths terminate at
>             separate ASBRs/forwarding addresses. In either case, each
>             path is represented by a separate routing table entry as
>             defined in Section 11.
>
>             This section only applies when RFC1583Compatibility is set
>             to "disabled".
>
>             The path preference rules, stated from highest to lowest
>             preference, are as follows. Note that as a result of these
>             rules, there may still be multiple paths of the highest
>             preference. In this case, the path to use must be determined
>             based on cost, as described in Section 16.4.
>
>             o   Intra-area paths using non-backbone areas are always the
>                 most preferred.
>
>             o   The other paths, intra-area backbone paths and inter-
>                 area paths, are of equal preference.
>
> ************************************************************************
> ********
> I don't quite understand what second bullet means, does it sugguest that
> OSPF will loadbalance between intra-area backbone paths and inter-area
> paths?
> Or it is totally implementation dependent?
>
> Thanks a lot,
> Jian
>
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