OSPF Costs Formula that include delay.

Jeff Tantsura jeff.tantsura at ericsson.com
Fri Jan 24 20:58:12 UTC 2014



1.OSPF (SPF) can only produce a SPT based on cost (metric).
Anything else would require CSPF rather than SPF.

2. Delay is not distributed as part of an IGP update
Typical constrains distributed are: bandwidth, color, some others

In IETF we are working to also be able to distribute those kinds of
metrics (draft-ietf-ospf/isis-te-metric-extensions)
draft-ietf-mpls-te-express-path defines how to use these metrics for
RSVP-TE (computation result is an ERO) however theoretically nothing
precludes one (implementation) to use those  for more comprehensive
computation, i.e. delay could be taken into consideration as long as the
path is loop free. So it would look like - compute all loop free paths to
a destination and then choose one with the smallest cumulative delay.

BTW - segment routing will give you this functionality day one :)


-----Original Message-----
From: Erik Sundberg <ESundberg at nitelusa.com>
Date: Friday, January 24, 2014 12:26 PM
To: Randy <randy_94108 at yahoo.com>, "nanog at nanog.org" <nanog at nanog.org>
Subject: RE: OSPF Costs Formula that include delay.

>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.
>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
>-----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)???
>> Thanks in advance
>umm..are you sure your question is not about EIGRP?
>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
>A vendor specific list would be a better place to ask.
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