The difference between root path cost and local path overhead in STP

The root-path overhead is based on the network segment between the switches, and the cost of the local path is also called the port overhead for the local physical link to the relevant IETF provisions for

Physical link Overhead

10G/S 2

1G/S 4

100m/s 19

10M/S 100

According to the picture above, SW1 is the root bridge, that SW2 F0/1 is the root port, because SW2 to from the network Segment 1 and network segment 3 received BPDU respectively, network segment 1 received root path cost is 0, local cost is 19, from the network Segment 3 received is the root path overhead is 19, local cost is 19. The 19 cost for that 100M is local overhead, and the root path overhead for the switch between the direct network segment, the reason that the network segment 3 of the root path cost is 19 is not 0 of the reason is that, network segment 3 BPDU for SW2, is passed the SW3, so SW2 f0/ The 2-port root-path overhead is added to the total cost of network Segment 2, which can be seen by show Spanning-tree brief, where the cost on the left is for the local port overhead, and the cost on the right is for the path overhead.

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