The metric that exists in a summary LSA is representative of the entire path
to the destination network, excluding situations where aggregation has taken
place. Specifically, when an ABR generates a type 3, it populates the
metric field of that summary with the current metric for the route as found
in the ABR's routing table. Hence, internal routers make informed
decisions regarding where to direct traffic flows. Again, aggregation may
tends to hide specific details.
I highly suggest a read of RFC 2328, particularly section 12.4.3 which
describes the algorithm for type 3 generation in great detail.
Pete
*********** REPLY SEPARATOR ***********
On 8/15/2001 at 10:20 AM Wilson, Bradley wrote:
>The question that's on my mind is where you have an area which has
>multiple
>ABRs. Do the internal routers simply compare the metrics to the respective
>ABRs and make their routing decision based on that comparison?
>
>BJ
>
>
>
>-----Original Message-----
>From: Howard C. Berkowitz [mailto:[EMAIL PROTECTED]]
>Sent: Wednesday, August 15, 2001 9:44 AM
>To: [EMAIL PROTECTED]
>Subject: Re: OSPF Distance Vector in the backbone? [7:16120]
>
>
>While I agree completely with Peter's statements, I think there may be two
>issues being mingled.
>
>Area 0.0.0.0, especially when there are no backbone-only routers, uses a
>DV-like algorithm to
>propagate inter-area and exterior routes. There's no use for a Dijkstra.
>
>Inside a nonzero area, the Dijkstra algorithm only computes intra-area
>routes, with a computational
>workload on the order of the square of the number of routes plus the
>logarithm of the number of routers.
>Inter-area and external routes are added to the routing table of that area
>as a second step, the workload for
>which is linear with the number of non-intra-area routes.
>
>At 08:55 AM 8/15/2001 -0400, you wrote:
>>Hey Ralph,
>>
>>This statement is quite true. Is there an area you wish to break down
>more
>>fully?
>>
>>For support, see the draft-ietf-ospf-abr-alt-04.txt which includes the
>>following text:
>>
>>In OSPF domains the area topology is restricted so that there must be
>> a backbone area (area 0) and all other areas must have either
>> physical or virtual connections to the backbone. The reason for this
>> star-like topology is that OSPF inter-area routing uses the
>> distance-vector approach and a strict area hierarchy permits
>> avoidance of the "counting to infinity" problem. OSPF prevents
>> inter-area routing loops by implementing a split-horizon mechanism,
>> allowing ABRs to inject into the backbone only Summary-LSAs derived
>> from the intra-area routes, and limiting ABRs' SPF calculation to
>> consider only Summary-LSAs in the backbone area's link-state
>> database.
>>
>>
>>*********** REPLY SEPARATOR ***********
>>
>>On 8/15/2001 at 12:12 AM Ralph Fudamak wrote:
>>
>> >Question about OSPF and LSA type 3 behavior. Doyle in Routing TCP/IP
>vol
>> >1:
>> >
>> > "When another router receives a Network Summary LSA from an ABR, it
>> >does
>> >not run the SPF algorithm. Rather it simply adds the cost of the route
>to
>> >the ABR and the cost included in the LSA. A route to the advertised
>> >destination, via the ABR, is entered into the route table along with the
>> >calculated cost. This behavior - depending on an intermediate router
>> >instead of determining the full route to the destination - is distance
>> >vector behavior. So, while OSPF is a link state protocol within an
>area,
>> >it
>> >uses a distance vector algorithm to find inter-area routes." (pg
>474,475)
>> >
>> >Please enlighten me.
>> >
>> >TIA,
>> >Ralph
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=16195&t=16120
--------------------------------------------------
FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html
Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
