OSPF Cost

I'm racking my head trying to make sense out of this ...

The book (Odom-ICND2) using the formula to calculate cost as:

ref-bw / Int-bw = OSPF cost

It's only a brief paragraph, but so far it's just not adding up for me

. He also uses the example of serial interfaces defaulting to 1544, which is rounded down to 64??? I can't even get that to add up on the old calculator. I'm sure if I can get the math, I will eventually master it... but I'm just not seeing it right now, any help would be mucho appreciados!

The reason I ask is the boson practice exam has a question on it that ask, about...

Which of the following statments is true about how OSPF derives the cost of an interface.

Well there are two options that contend if the ref-bw were changed using the auto-cost reference-bandwidth 1000 or auto-cost reference-bandwidth 100 would that change increase the previously calculated cost by a factor of 10?

Naturally I pulled out the book to reference the section, but I'm still lost on this calculation.

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Comments

jason_lunde

Here man, the top two questions should clear it up for you.
OSPF: Frequently Asked Questions - Cisco Systems

HTH's

Oh, and on the serial thing.... that is what a serial link defaults to, 1.544 Mbps. So if you take the default 100Mbps as the referenced bw, and divide it by 1.544 (link bw) you get 100/1.544 = 64...and some change

fieldmonkey

jason_lunde wrote: »

Oh, and on the serial thing.... that is what a serial link defaults to, 1.544 Mbps. So if you take the default 100Mbps as the referenced bw, and divide it by 1.544 (link bw) you get 100/1.544 = 64...and some change

Ok ...I don't think I've been reading it right or something. I read the link you provided (thanks) and it seems more simply put than the book or it could just be me probably.

So if the ref-BW is always 100,000 (10Mbps) which would never change unless you used the auto-cost reference-bandwidth command. On a FE interface running 100 Mbps the cost would equal 1.

I know it's simple math, but for whatever reason I got to thinking 10 Mbps was the ref-BW and the book started using kbps, introducing the 1544 and at the same time the page needed to be turned to keep reading ... it was time to put this one on ice til today. I was calculating all backwards, but now I have it! Thanks!

10 Mbps | 100,000 / 10,000 = 10
100 Mbps | 100,000 / 100,000 = 1
1000 Mbps | 100,000 / 1,000,000 = .1 (why does the cost still equal 1???, according to the book... hrmmm)
T1 line | 100,000 / 1544 = 64.7

jason_lunde

When you get into using speeds above gig, it might be time to use the auto-cost reference-bandwidth command. Depending on organizational policies, one might want to develop a schema that will fit their organizations routing needs.

Aldur

fieldmonkey wrote: »

1000 Mbps | 100,000 / 1,000,000 = .1 (why does the cost still equal 1???, according to the book... hrmmm)

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.

fieldmonkey

Aldur wrote: »

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.

jason_lunde wrote: »

When you get into using speeds above gig, it might be time to use the auto-cost reference-bandwidth command.

Ok, so I gather this ...

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?

That's kind of like playing God, isn't it?

Thanks so much for helping me get a grip on this one!

jason_lunde

Aldur wrote: »

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.

Nice to see you in this neck of the woods Aldur. How far are you going to go on the cisco journey? All the way

ColbyG

fieldmonkey wrote: »

Ok, so I gather this ...

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?

Yes. If you have links faster than 100mb in your OSPF domain you should change the reference bandwidth to reflect this.

Aldur

fieldmonkey wrote: »

Ok, so I gather this ...

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?

Exactly, sounds like you understand it perfectly now

jason_lunde wrote: »

Nice to see you in this neck of the woods Aldur. How far are you going to go on the cisco journey? All the way

Heh, yea, I'm shooting for the CCIE-SP for a long term goal. Gonna get my CCNA out of the way in a month or two, recert my JNCIE-M/T and then hopefully pick up the JNCIE-SEC if it comes out this year. Then back to the cisco studies... I have a very understanding wife

Dilbert65

Here is a little chart that contains the main connection speeds you will run across.

56k = 1785
T1 = 64
10BaseT = 10
FDDI/ 100BaseT = 1

fieldmonkey

Dilbert65 wrote: »

Here is a little chart that contains the main connection speeds you will run across.

56k = 1785
T1 = 64
10BaseT = 10
FDDI/ 100BaseT = 1

Thanks Dilbert... I can only hope that one day I will be considered enough of an asset to an IT Manager / Network Engineer, enough to utilize that list one day! ... lol

Being a fieldmonkey is ok, but I want more of a challenge, without having to take such a steep paycut. I have a little commute too, so I can't do it for the entry level salary.