Summary Post – Methods to Manipulate OSPF Costs

There are three ways to manipulate the interface cost in OSPF.  One is very direct, one changes the presentation of the interface, and the other changes the calculations for every interface.

Set the cost of the interface directly – Just give it the number you want.  Easy.  This is the number OSPF will use in the SPF calculations without doing any math on the interface.

Set the bandwidth of the interface – The formula that OSPF uses to calculate interface cost is pretty easy to remember – (reference bandwidth) / (interface bandwidth).  Changing the interface bandwidth will obviously change the result of the calculation.  The same caveat for EIGRP route manipulation holds true here; if you change the bandwidth of the interface, you may affect other things like QoS…or EIGRP, now that I mention it.

Set the reference bandwidth – This changes the numerator of the calculation.

Notice the nice message about making the reference bandwidth consistent.  If you were to only set the reference bandwidth on this one routers, then each router would calculate the path costs differently.  That will lead to undesirable results, I imagine.

Send any SPF calculation overhead questions to me.

Aaron Conaway

I shake my head around sometimes and see what falls out. That's what lands on these pages.

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