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STP uses the extended system ID plus a MAC address to make the bridge ID unique for each VLAN.
If another bridge in the same spanning tree domain does not run the MAC address reduction feature, it
could achieve root bridge ownership because its bridge ID may fall between the values specified by the
MAC address reduction feature.
Note
Understanding BPDUs
Switches transmit bridge protocol data units (BPDUs) throughout the STP instance. Each switch sends
configuration BPDUs to communicate and compute the spanning tree topology. Each configuration BPDU
contains the following minimal information:
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The unique bridge ID of the switch that the transmitting switch determines is the root bridge
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The STP path cost to the root
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The bridge ID of the transmitting bridge
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Message age
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The identifier of the transmitting port
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Values for the hello, forward delay, and max-age protocol timer
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Additional information for STP extension protocols
When a switch transmits a Rapid PVST+ BPDU frame, all switches connected to the VLAN on which the
frame is transmitted receive the BPDU. When a switch receives a BPDU, it does not forward the frame but
instead uses the information in the frame to calculate a BPDU, and, if the topology changes, initiate a BPDU
transmission.
A BPDU exchange results in the following:
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One switch is elected as the root bridge.
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The shortest distance to the root bridge is calculated for each switch based on the path cost.
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A designated bridge for each LAN segment is selected. This is the switch closest to the root bridge
through which frames are forwarded to the root.
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A root port is selected. This is the port providing the best path from the bridge to the root bridge.
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Ports included in the spanning tree are selected.
Cisco Nexus 3000 NX-OS Layer 2 Switching Configuration Guide, Release 5.0(3)U3(1)
66 OL-26590-01
Configuring Rapid PVST+
Understanding STP