From: Paul Cosgrove (paul.cosgrove@heanet.ie)
Date: Sat Mar 15 2008 - 08:50:01 ARST
When calculating the best inter-area route, an OSPF ABR which has a
neighbor in the backbone will not consider any summary LSAs learned from
a non-backbone area. If it does not have a neighbor in the backbone,
then it will accept the summary LSAs from another area. Cost is checked
later.
Paul.
Mike Haddad wrote:
> Hello Malik,
>
> OSPF uses SPF algorithm to calculate the route to destinations. SPF is
> calculated in three phases. The first is the calculation of intra-area routes
> by building the shortest path tree for each attached area. The second phase
> calculates the inter-area routes by examining the summary LSAs and the last
> one examines the AS-External-LSAs to calculate the routes to the external
> destinations.
>
> In your example, R2 will perform the SPF calculation on both received
> summary routes and will choose the one that has the lowest metric => shortest
> path. You can manipulate the route selection by many ways and one is by
> increasing the OSPF Cost on the interface going to R3 so it uses R1 for the
> summary.
>
> Hope this helps,
>
> Regards,
>
>> Date: Sat, 15 Mar 2008 01:53:08 -0400> From: ccie.09@gmail.com> To:
>>
> ccielab@groupstudy.com> Subject: Route selection for Inter Area routes> >
> All,> I would appreciate if some one can explain how inter area routes are>
> selected in the following scenario> > R2 is receiving type-3 summary routes
> from an ABR R1 in back bone area and> receiving the same route from R3 through
> area say 10.> > > R2 & R3 are neighbors in area 10 and they both are part of
> back bone also.> > Thanks> Malik> >
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