Bpdus - Cisco Nexus 7000 Series Configuration Manual

Configuring Rapid PVST+ Using Cisco NX-OS
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STP uses the extended system ID plus a MAC address to make the bridge ID unique for each VLAN.
Note If another bridge in the same spanning tree domain does not run the MAC address reduction feature, it
could win the root bridge ownership because of the finer granularity in the selection of its bridge ID.

BPDUs

Network devices transmit BPDUs throughout the STP instance. Each network device 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 network device that the transmitting network device believes to be the root
bridge
• The STP path cost to the root
• The bridge ID of the transmitting bridge
• The 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 network device transmits a Rapid PVST+ BPDU frame, all network devices connected to the VLAN
on which the frame is transmitted receive the BPDU. When a network device receives a BPDU, it does not
forward the frame but instead uses the information in the frame to calculate a BPDU. If the topology changes,
the device initiates a BPDU exchange.
A BPDU exchange results in the following:
• One network device is elected as the root bridge.
• The shortest distance to the root bridge is calculated for each network device based on the path cost.
• A designated bridge for each LAN segment is selected. This network device is closest to the root bridge
through which frames are forwarded to the root.
Cisco Nexus 7000 Series NX-OS Layer 2 Switching Configuration Guide, Release 5.x
STP
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