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=16175&t=16120
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