Notice that STP terminology refers to the devices as bridges rather than switches. Recall (from Appendix B) that bridges are previous-generation devices with the same logical functionality as switches; however, switches are significantly faster because they switch in hardware, whereas bridges switch in software. Functionally, the two terms are synonymous.
Within an STP network, one switch is elected as the root bridgeit is at the root of the spanning tree. All other switches calculate their best path to the root bridge. Their alternate paths are put in the blocking state. These alternate paths are logically disabled from the perspective of regular traffic, but the switches still communicate with each other on these paths so that the alternate paths can be unblocked in case an error occurs on the best path.
All switches running STP (it is turned on by default in Cisco switches) send out bridge protocol data units (BPDUs). Switches running STP use BPDUs to exchange information with neighboring switches. One of the fields in the BPDU is the bridge identifier (ID); it is comprised of a 2-octet bridge priority and a 6-octet MAC address. STP uses the bridge ID to elect the root bridgethe switch with the lowest bridge ID is the root bridge. If all bridge priorities are left at their default values, the switch with the lowest MAC address therefore becomes the root bridge. In Figure 2-4, switch Y is elected as the root bridge.
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