Re: OSPF cost - LAN segment

 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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Received on Thu May 21 2009 - 08:03:14 ART