Hello my friend.
STP use COST to choose correct ports role. Look text below;
When the root bridge has been elected for the spanning-tree instance, STA starts determining the best paths to the root bridge.
Switches send BPDUs, which include the root path cost. This is the cost of the path from the sending switch to the root bridge. It is calculated by adding the individual port costs along the path from the switch to the root bridge. When a switch receives the BPDU, it adds the ingress port cost of the segment to determine its internal root path cost. It then advertises the new root path cost to its adjacent peers.
The default port cost is defined by the speed at which the port operates. As shown in Table 3-1, 10 Gbps Ethernet ports have a port cost of 2, 1 Gbps Ethernet ports have a port cost of 4, 100 Mbps Fast Ethernet ports have a port cost of 19, and 10 Mbps Ethernet ports have a port cost of 100.
Table 3-1 Revised IEEE Cost Values
| Link Speed |
Cost (Revised IEEE 802.1D-1998 Specification) |
| 10 Gbps |
2 |
| 1 Gbps |
4 |
| 100 Mbps |
19 |
| 10 Mbps |
100 |
Jaderson Pessoa
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