From: Paul Cosgrove (paul.cosgrove@heanet.ie)
Date: Sun Mar 16 2008 - 05:51:21 ART
Better correct myself here: the rule mentioned below is not the only
summary lsa rule which in practice prevents disconnected backbones being
used whilst retaining full connectivity, just one of them.
Paul Cosgrove 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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