Specifying OSPF cost and bandwidth

I'm somewhat confused by the fact that both cost and bandwidth can be configured in OSPF. I realise that there's a simple relationship between them - cost = 100,000,000/bandwidth. I've just played around with Packet Tracer and found that, if I have 2 routers connected via an ethernet cable, I can set either cost or bandwidth. When I change bandwidth, the cost changes automatically but when I change the cost, the bandwidth doesn't change.

In a real life situation, what are the circumstances when I should change cost rather than bandwidth and what are the circumstances when I should change bandwidth rather than cost?

I'd add that I'm towards the beginning of CCNA3 so, if the answer means lots of knowledge about the WAN technologies in CCNA4, I'm happy to have a simple answer like "It's related to the configuration of WAN components and all will become clear in the next module"!

Thanks for your time.

The Bandwidth command is useful if the data throughput from the interface is less than its physical capacity - that is, the actual throughput should distort the cost for your network but doesn't. If you have a WAN connection, say a fibre connection which terminates on a gigabit ethernet interface but you only purchase 20mb of bandwidth from the provider then the cost (1 iirc) would be misleading in your topology since you can only ever push 20mb of traffic down the link.

Very valid scenario novembre, further more:

I can set either cost or bandwidth. When I change bandwidth, the cost changes automatically but when I change the cost, the bandwidth doesn't change.

The cost is the final player that is used in OSPF for path cost determination. While bandwidth is meant to be a pure measure of the link max capacity (or allowed capacity). My guess is that Cisco made changes to bandwidth reflect on cost simply for convenience. But the other way around does not (changes to cost does not reflect on bandwidth) to allow you to set a custom cost that you want to use regardless of the given bandwidth.

Simply speaking, if the calculated cost of a link was 1. And you want to use a cost say 10 to force OSPF not to choose this link while routing, then that does not mean that the bandwidth has changed.

Hope that makes sense.