OSPF and default reference bandwidth

Answered Question
Jul 23rd, 2008

OK, so OSPF uses 100mb as a reference bandwidth by default and divides that by the interface bandwidth to create the cost for that route.

Is the reference bandwidth only locally significant to the local router, or is it globally significant.

So if I wanted to take into account the bandwidth for gig and 10 gig interfaces, can I change the reference bandwidth only on the devices with that type of interface or should I change it globally on all routers?

I have this problem too.
0 votes
Correct Answer by lee.reade about 8 years 4 months ago

Hi,

The value is locally significant, however best practice is to config the same value for autocost-ref-bandwidth across the board.

This will ensure that you dont get any funny routing issues, with some router choosing one path and another choosing a different return pth, ie asymetrical routing.

HTH

LR

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Correct Answer
lee.reade Wed, 07/23/2008 - 10:51

Hi,

The value is locally significant, however best practice is to config the same value for autocost-ref-bandwidth across the board.

This will ensure that you dont get any funny routing issues, with some router choosing one path and another choosing a different return pth, ie asymetrical routing.

HTH

LR

petersonb Wed, 07/23/2008 - 11:48

Hi,

Thanks that pretty much answers my question.

I guess the follow-up would be is how useful is it to change the reference bandwidth?

Right now for us we are looking to move to a 10Gig core / distribution on our campus, so I can see the benefit there when choosing multiple paths across the campus, but across the WAN it really would not make a difference for us.

Thanks,

Ben

Edison Ortiz Wed, 07/23/2008 - 10:59

The change is locally significant but failing to change this value in all routers in the network can produce some asymmetrical routing.

For instance, your router may prefer the 10Gbps interface under the new setting but the neighboring router may prefer the 100Mbps interface with the old setting. Consistency is the key.

HTH,

P.S. Sorry Lee, I didn't see your post while replying :)

__

Edison.

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