OSPF path selection ??

[selva Kathir]

Hi,

I have the below topology in my network I am planing to configure ospf in my network .

Can any one confirm that how traffic will flow from B router to reach the networks behind the a router .

with default ospf configurations .

will it take the directly connected link to reach the router A 's network network ? or will it go through router C to reach the router A ?

Thanks ,

[selva Kathir]

Below is my topology

[Akash Agrawal · Accepted solution]

Hi Selva,

If you have default OSPF configuration. It will do load balance.

Cost of link connecting Router-A and Router-B = 10^8/1mb = 100

Cost of link connecting Router-B and Router-C = 10^8/2mb = 50

Cost of link connecting Router-C and Router-A = 10^8/2mb = 50

So router B is having two equal cost path to reach A.

Regards,

Akash

[selva Kathir]

Hi Akash ,

thanks for your response .

In this case will ospf  give a priority to the directly connected link  ???

Thanks ,

Selva

[Akash Agrawal]

Hi Selva,

For OSPF it is only metric , based on which it decides best path. No preference for directly connected link.

Regards,

Akash

[Prashant Sheshasayee]

ospf select the path basis on the cost.It will also use load balancing if the config is default.

[Joseph W. Doherty]

Disclaimer

The   Author of this posting offers the information contained within this   posting without consideration and with the reader's understanding that   there's no implied or expressed suitability or fitness for any purpose.   Information provided is for informational purposes only and should not   be construed as rendering professional advice of any kind. Usage of  this  posting's information is solely at reader's own risk.

Liability Disclaimer

In   no event shall Author be liable for any damages whatsoever (including,   without limitation, damages for loss of use, data or profit) arising  out  of the use or inability to use the posting's information even if  Author  has been advised of the possibility of such damage.

Posting

I just wanted to add what Akash describes is correct assuming normal default setup on Cisco OSPF devices for path costing.  However, if costing isn't using defaults traffic might flow directly from B to A or B to C to A or still equally via both paths.

OSPF uses the interface costs from (each) router to destination.  Costs per interface might be set manually, or on some vendors equipment (including Cisco) they may autocost based on interface's bandwidth.  Different vendors, when they autocost, might use different computations for the costing and Cisco allows you to modify the base value (NB: 100 Mbps, by default).

So, to truly answer your question, we would need to know the OSPF cost across each link, not the bandwidth.  (Again, though, assuming Cisco defaults, Akash's answer is correct.)

[selva Kathir]

thanks joseph and akash

for your answers .