Re: OSPF cost - LAN segment

Right. i missed FastE/E in the diag there. But DCE/DTE of serial should also
be taken in consideration

On Thu, May 21, 2009 at 8:03 AM, Abraham, Tharak <tharakabraham_at_gmail.com>wrote:

> Hi Welly,
>
> Neat diag. there.
>
> By default on an OSPF network
>
> cost of loopback is 1
> cost of ethernet is 10
> cost of serial line is 64
> cost of fast ethernet is 1
>
> The above is derived based on the formula 10^8 / BW (bps)
> The cost is associated with the interface OSPF uses and inversely
> propotional to the bandwidth of the interface
>
> When traversing the link from R1 to R2 (viewing from R1) we will go in the
> forward direction, ie R1 has to consider the cost of the Fa on itself and
> the Serial interface and the cost of the loopback interface on R2 to reach
> there and the
> cumilative cost adding to 1 + 64 + 1 = 66.
>
> On the reverse, from R2 to R1 (viewing from R2) in the forward direction
> the
> cost to reach R1's loopback is 64 + 10 + 1 = 75
>
> So the answer to your question when running OSPF, R1 and R3 sees a
> different
> costs from its own individual view.
>
> Making it simpler, its better if you have the "tree view" from the
> respective router and consider the costs of the exiting/outgoing interface
> of the route and just jump hop by hop to reach your final destination and
> the total will give you the metric/cost of the route.
>
> i hope that made some logic/sense..-:)
>
> Best Regards,
> Tharak.
>
> On Thu, May 21, 2009 at 5:06 AM, Welly Kamarudin <welly.wewe_at_gmail.com
> >wrote:
>
> > Hi all,
> >
> > May I know a little bit detail about ospf cost,
> >
> > For example I have topology like below
> >
> > R1 (fa0/0) |------ SW1------| (e0/0) R3 (s1/0) ======= (s1/0) R2
> > cost ?
> >
> > ------| ethernet segment
> > === serial interfaces
> >
> > Using default bandwidth reference.
> > I want to ask what will be the cost on the LAN segment between R1 and R3
> > Is it true that, there will be two different costs there depending on
> from
> > which point we look the routing table?
> > So for example I put loopback on R2 and R1.
> > The cost from R1 when try to reach loopback on R2 will be lower if I
> > compare
> > with the cost from R2 when try to reach loopback R1.
> >
> >
> > Thanks for clarifying
> > Welly
> >
> >
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