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....