# OSPF Costs Formula that include delay.

Erik Sundberg ESundberg at nitelusa.com
Fri Jan 24 20:26:39 UTC 2014

```I understand OSPF default calculation for cost doesn't include delay. I am looking for a formula that I can use to manually set the OSPF costs that factors in delay.

When using OSPF's default costs, the shortest path is not always the optimal path.

Example

New York to Los Angeles. Assuming all links are the same bandwidth and have a ospf cost of 1.

Path 1 (75ms) - OSPF Cost 2 - New York > Dallas > Los Angeles

Path 2 (65ms) - OSPF Cost 3 - New York > Chicago > Denver > Los Angeles

If I left the default cost's alone then path 1 would win because it has a lower ospf cost, however it take traffic 10ms longer to get there.

However I would like traffic to take Path 2 by adjusting the OSPF cost.

I am looking for a formula that other people are using .p

Thanks

Erik

-----Original Message-----
From: Randy [mailto:randy_94108 at yahoo.com]
Sent: Thursday, January 23, 2014 9:03 PM
To: Erik Sundberg; nanog at nanog.org
Subject: Re: OSPF Costs Formula that include delay.

----- Original Message -----
> From: Erik Sundberg <ESundberg at nitelusa.com>
> To: "nanog at nanog.org" <nanog at nanog.org>
> Cc:
> Sent: Thursday, January 23, 2014 4:47 PM
> Subject: OSPF Costs Formula that include delay.
>
> What is everyone using for an OSPF cost formula that factors in a
> circuits delay and bandwidth (10M-100G)???
>

OSPF has no concept of interface-delays.

The default reference bandwidth for OSPF is 100M

In your case if you set your reference bandwidth to 100000 your 100G links would have a link cost of 1, 10G - 10, 1G-100, 100M-1000 and 10M-10000

A vendor specific list would be a better place to ask.

./Randy

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