The way that STP chooses the root bridge can cause an interesting situation if left to the default values. Recall that the MAC address is a 6-octet or 48-bit value, with the upper 24 bits as an Organizational Unique Identifier (OUI) (representing the vendor of the device) and the lower 24 bits as a unique value for that OUI, typically the serial number of the device. A lower MAC address means a lower serial number, which likely means an older switch. Thus, because STP by default chooses a switch with a lower MAC address, the oldest switch is likely to be chosen. This is just one reason why you should explicitly choose the root bridge (by changing the priority), rather than getting the STP default choice.

All the ports on the root bridge are called designated ports, and they are all in the forwarding statethat is, they can send and receive data. (The STP states are described in the next section of this chapter.)

On all nonroot bridges, one port becomes the root port, and it is also in the forwarding state. The root port is the one with the lowest cost to the root. The cost of each link is by default inversely proportional to the bandwidth of the link, so the port with the fastest total path from the switch to the root bridge is selected as the root port on that switch. In Figure 2-4, port 1 on switch X is the root port for that switch because it is the fastest way to the root bridge.

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