Understanding Bpdus - Cisco AP775A - Nexus Converged Network Switch 5010 Configuration Manual

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Understanding BPDUs

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.

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:

• The unique bridge ID of the switch that the transmitting switch determines is the root bridge

• The STP path cost to the root

• The bridge ID of the transmitting bridge

• Message age

• The identifier of the transmitting port

• Values for the hello, forward delay, and max-age protocol timer

• 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:

• One switch is elected as the root bridge.

Cisco Nexus 5000 Series Switch CLI Software Configuration Guide

Information About Rapid PVST+

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