OSPF cost & L3 etherchannel

Unanswered Question
Feb 14th, 2008

If I bundle 3 gigabit links into a Layer 3 etherchannel, will ospf notice the bandwidth of 3Gbps when it calculates the ospf cost? or does it use 1 Gbp?

I have this problem too.
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lamav Thu, 02/14/2008 - 13:15


OSPF will definitely treat the EtherChannel as a 3 Gbps link and calculate the Cost accordingly.

Remember that OSPF calculates Cost based on a reference bandwidth of 10 to the 8 (100 million)/configured bandwidth. So, a 100 M FDDi connection defaults to a cost of 1. Anything higher than that will still default to a cost of 1 (since you cannot have a fractional Cost) and nothing will be gained by havin gthat extra bandwidth configured for the interface.

So, to accomodate high-speed links above 100M, use the 'auto-cost reference-bandwidth command while in router configuration mode or use the 'ip ospf-cost' command at the interface to hardcode the Cost.



cisconoobie Fri, 02/15/2008 - 07:03

I set the auto-cost reference bandwidth to 1000000 for future 10Gbps upgrades and the costs dont add up.

What would the formula be for the 3Gbps connection?

1,000,000/3,000 ?

lamav Fri, 02/15/2008 - 08:23


The formula that Cisco's OSPF implementation uses to determine a link's Cost is 100,000,000 (100M)divided by the reference bandwidth. You have typed 1 million divided by the reference bandwidth.

lamav Fri, 02/15/2008 - 09:34


Good catch. I didnt read his post the way you did. I though tthe final reference bandwidth he configured was 1,000,000 (1M), I didnt think that he may have actually entered "1000000" as the argument in the command itself.

Maybe that's what he did....


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