OSPF routing

[sshah329]

Hello Guys,

I think my old account was deleted or something happaned to it.

So my question is 1. How does OSPF calculate what is the cost of the link if it is > 100M, lets say we have two links 1g and 10g, the cost in each cases will be 1, right, 10^8/bw? how does ospf select which one to select and how?

2. I am sure there are some junos experts here, how does ospf in juniper calculate cost? is it the same bw?

Thanks,

SS

[Leo Laohoo]

OSPF Design Guide

[sshah329]

Hi, thanks for your replies,

I think we can also manually change the cost. havent tried that yet.

[Reza Sharifi]

Hi SS,

Juniper calculates the cost the same as Cisco devices.  It uses the same formula you provided

Now, if you want Junos to prefer a 10Gig link vs a 1 Gig link, you need to use this command:

set protocol ospf reference-bandwidth

More info:

http://www.juniper.net/techpubs/software/junos/junos94/swconfig-routing/reference-bandwidth.html#id-11227690

HTH

[Kishore Chennupati]

Reza marketing Junipers on cisco forums Hehee kidding.

[Joseph W. Doherty]

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So my question is 1. How does OSPF calculate what is the cost of the link if it is > 100M, lets say we have two links 1g and 10g, the cost in each cases will be 1, right, 10^8/bw? how does ospf select which one to select and how?

Cisco OSPF would, by default, would consider both the gig and 10 gig links (and 100 Mbps) as equals.  To have it do otherwise, you'll need to modify the metrics for the two links.  One method would be to change the default reference bandwidth from 100 Mbps to 10 gig.

[Ganesh Hariharan]

Currently Being Moderated
OSPF routing
Hello Guys, 
I think my old account was deleted or something happaned to it.
So my question is 1. How does OSPF calculate what is the cost of the link if it is > 100M, lets say we have two links 1g and 10g, the cost in each cases will be 1, right, 10^8/bw? how does ospf select which one to select and how?
2. I am sure there are some junos experts here, how does ospf in juniper calculate cost? is it the same bw?
Thanks,
SS

Hi SS,

Mathematically 1g should equal .1 but with reference bandwidth you can't go lower then 1. Hence the problem with the default settings if you have 100mb/s and 1gb/s links in your network, by default OSPF is going to see the two as the same cost and route accordingly.

A routers' IOS, relative to caculating OSPF cost, will never calculate a cost less than 1. So for an interface running above 100 Mbps, one would need to utlize the auto-cost reference-bandwidth command in order to take advantage of the difference in the size of the pipes.

Junos ios also uses the same type of cost calculation.

Hope to Help !!

Ganeshh Iyer

[sshah329]

Thanks guys for giving these answers, most of them helped

have a good one,