OSPF & Cost calculation...

r_durant

Cisco uses the equation (10^8/bandwidth) to determine the cost of an interface....

Hence, a 100Mbps FastEthernet would have a cost of 1 (100,000,000/100,000,000) and 10Mbps Ethernet, a cost of 10, etc...

These default values can be overridden using the ip ospf cost command and the value must be in the range of 1 to 65,535.

My questions....Does Cisco's implementation of OSPF support Gbps interfaces? Since the cost would be 0.1? And it seems the cost must be => 1.

Todd's book doesn't mention it, but it's say an interface set with a bandwidth of 64,000 would have a default cost of 1563, but in the math world 100,000,000/64,000 is actually 1562.5.

I did some googling, but couldn't find anything to support....

It's just something I'm curious about...

r_durant

Wow...decided to run upstairs and check my ICND book (I was too lazy at first... )

And bang, right there it says...For Gbps interfaces. it yields a cost of .1, but only interger values can be used, so OSPF uses a cost of 1 for Gbps interfaces as well.....

It also goes on to say you can manipulate the equation using the auto-cross reference-bandwidth 1000 command and change the numerator to 10^9, which would make the cost of a Gbps interface 1 and a FastEthernet would be 10.....NICE!!

Goes to show how you "may" need both books after all....

I gues I answer my own question....

Netstudent

I was going to say change the multiplier or cost reference but its good to know Gb defaults to 1. It seems that you would have to be very mindful of what that can do to your network though.

r_durant

Yea, Todd's book didn't go into it, so the first thing that came to mind was...What about Gbps?

It's also good to have more than one source of info as well....