Cost

We need a metric for each link in a network and the standard way to do this in OSPF is to consider the apparent bandwidth of a link; the faster the link the lower the cost. The cost of a link is:

For example, a 100Mb/s link is the reference link and will have a cost of 1.

A 10Mb/s link will have a cost of 10.

So why do the calculation?

Remember, OSPF advertises link-state, so we need to pass these values around the network in LSAs. By dividing the speed like this, we can send a small number. However, link cost can also be set manually by the administrator.

Remember that the overall cost between two points is the aggregate cost of all the links between two points.