OSPF Reference Bandwidth

This topic is to discuss the following lesson:

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well explained.

“It now has a cost of 1 which means that a Gigabit interface would end up with a cost of 1.”

Did you mean 10? :wink:

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Oops yes :slight_smile: Just fixed it! Thanks Scott.

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check this command its not working

Router#show ip ospf | include Reference
 Reference bandwidth unit is 100 mbps

this one is working

Router#show ip protocols | include Reference

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Hey Rene,
I’ve seen auto-cost reference-bandwidth as well as reference-bandwidth.
Is there any difference between those two commands or are they just IOS version dependent?
Thank you!

I have only seen auto-cost reference-bandwidth for OSPF (both in IOS 12 and 15). There is a reference-bandwidth command, but it is in the NX-OS and unrelated to OSPF.

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well explained

HI Rene,

Do you have to set the reference bandwidth on both sides of a link? Is there a way of distributing a reference bandwidth throughout an OSPF area?

Hi Max,

There is no way to synchronize the reference bandwidth so you’ll have to configure it yourself.

It’s not mandatory to use the same metric on both sides, you could use different metrics if you want.

i am little bit confused in refrence bandwidth
1> if i have gigabyte link in my side router and other side router also has a gigabyte link from which they are connected so i will set reference bandwidth to 1000mbps on both side router its compulsory
2> if i have gigabyte link in my side router and other side router has a fastethernet link from which they are connected so in this topology what we will do we will change the reference bandwidth on fastethernet router or not
(((Cisco IOS will warn you that you should do this on all OSPF routers ))) what about this line means

Hello Harshit

Fundamentally, reference bandwidth should always be the same on all routers taking part in OSPF. Now if you choose to use 1000 Mbps as the reference bandwidth or 10000Mbps, or even 100 Mbps, it doesn’t make a difference what the speeds on your interfaces are, as long as all routers are choosing costs based on the same reference bandwidth.

Now in the case where you have a GigabitEthernet interface connected to a FastEthernet interface, it shouldn’t make a difference. This is because if these two interfaces were connected, the GigabitEthernet link would revert to to 100Mbps, and the reference bandwidth used would refer to that speed rather than the GigabitEthernet speed. So even if you use 100Mbps or 1000Mbps for the reference bandwidth, the result would still be good.

Rule of thumb: always have the same reference bandwidth on all OSPF routers.

I hope this has been helpful!