From: S Malik (ccie.09@gmail.com)
Date: Sat Mar 15 2008 - 23:32:53 ART
Great explanation.
Thanks, I appreciate it.
On 3/15/08, Paul Cosgrove <paul.cosgrove@heanet.ie> wrote:
>
> Hi Malik,
>
> It is specified in the RFC (2328):
>
> 16.2. Calculating the inter-area routes
>
> The inter-area routes are calculated by examining summary-LSAs.
> If the router has active attachments to multiple areas, only
> backbone summary-LSAs are examined.
>
> Incidently, it is this rule which imposes the practical restriction that
> you cannot have separated backbone areas. There are actually some
> scenarios where you can have disconnected backbone areas and retain full
> connectivity, provided they have only one router in each. However the
> summary LSA rules mean that these scenarios are very limited in size and
> do not handle link failures well (so don't have much practical use).
>
> Paul.
>
> S Malik wrote:
> > Paul/Mike,
> > Thanks for your assistance.
> >
> > I found exactly the same behaviour. A route received by a neighbor in BB
> is
> > always selected regardless of metric. Once I create a virtual link
> through
> > area 10 and manipulate metrics then route is chosen through router R3.
> > I think, I was not clear that BB route is always preferred. Can any one
> > point me to any link explaining this behaviour please. Most of docs
> explain
> > only intra-area, inter-area, E1, E2 only.
> >
> > I love this forum with great participants.
> >
> > Malik
> >
> >
> > On Sat, Mar 15, 2008 at 5:50 AM, Paul Cosgrove <paul.cosgrove@heanet.ie>
> > wrote:
> >
> >
> >> 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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