Processing Superior Bpdu Information; Processing Inferior Bpdu Information; Blocking State - Cisco Nexus 7000 Series Configuration Manual

Configuring Rapid PVST+ Using Cisco NX-OS
If a designated port is in the forwarding state and is not configured as an edge port, it transitions to the blocking
state when the Rapid PVST+ forces it to synchronize with new root information. In general, when the Rapid
PVST+ forces a port to synchronize with root information and the port does not satisfy any of the above
conditions, its port state is set to blocking.
After ensuring that all of the ports are synchronized, the device sends an agreement message to the designated
device that corresponds to its root port. When the devices connected by a point-to-point link are in agreement
about their port roles, Rapid PVST+ immediately transitions the port states to the forwarding state.
This figure shows the sequence of events during synchronization.
Figure 9: Sequence of Events During Rapid Convergence

Processing Superior BPDU Information

A superior BPDU is a BPDU with root information (such as a lower switch ID or lower path cost) that is
superior to what is currently stored for the port.
If a port receives a superior BPDU, Rapid PVST+ triggers a reconfiguration. If the port is proposed and is
selected as the new root port, Rapid PVST+ forces all the designated, nonedge ports to synchronize.
If the received BPDU is a Rapid PVST+ BPDU with the proposal flag set, the device sends an agreement
message after all of the other ports are synchronized. The new root port transitions to the forwarding state as
soon as the previous port reaches the blocking state.
If the superior information received on the port causes the port to become a backup port or an alternate port,
Rapid PVST+ sets the port to the blocking state and sends an agreement message. The designated port continues
sending BPDUs with the proposal flag set until the forward-delay timer expires. At that time, the port transitions
to the forwarding state.

Processing Inferior BPDU Information

An inferior BPDU is a BPDU with root information (such as a higher switch ID or higher path cost) that is
inferior to what is currently stored for the port.
Cisco Nexus 7000 Series NX-OS Layer 2 Switching Configuration Guide, Release 5.x
Rapid PVST+
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