Path Cost and Root Path Cost

Path costs are important to determine the fastest link from a switch to another. There are actually 2 types of path cost in STP which is Path Cost and Root Path Cost. Path cost are costs from one switch to another neighbouring switch. This is what is usually been shown next to the interface to the neigbour switch as shown below.

Root Path Cost are all the costs added up from a switch to reach its Root Bridge. This costs are locally significant and are not known by the downstream switches. For example if there are 3 switches (Switch A – root, B – middle and C – lowest) connected in serial to each other with the cost of 19 each, then the Path Cost of A to B is 19, the Path Cost of B-C is 19, but there is no Path Cost from A-C because it is not directly connected. The Root Path Cost of A is 0 (because it is the root), RPC of B is 19 (needs to go 1 hop to reach switch A) and RPC of C is 19+19=38 (needs to go 2 hops with each hop with a link cost of 19).

The costs of the link between the neighbouring switches are determined by the speed of the port. These costs are:

Port	Original Cost	Current Cost
10 Mbps	100	100
100 Mpbs	10	19
1 Gbps	1	4
10 Gbps	1	2