From: Paul Cosgrove (paul.cosgrove@heanet.ie)
Date: Sat Mar 15 2008 - 22:09:26 ARST
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> >
>>> _______________________________________________________________________>
>>> Subscription information may be found at: >
>>> http://www.groupstudy.com/list/CCIELab.html
>>> _________________________________________________________________
>>> Spread the Love by installing 30 free Messenger Emoticons. Get them now!
>>> http://g.msn.ca/ca55/212
>>>
>>> _______________________________________________________________________
>>> Subscription information may be found at:
>>> http://www.groupstudy.com/list/CCIELab.html
>>>
>
> _______________________________________________________________________
> Subscription information may be found at:
> http://www.groupstudy.com/list/CCIELab.html
This archive was generated by hypermail 2.1.4 : Tue Apr 01 2008 - 07:53:53 ART