Detecting Unidirectional Link Failure - Cisco AJ732A - MDS 9134 Fabric Switch Configuration Manual

Information About Rapid PVST+
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After ensuring that all of the ports are synchronized, the switch sends an agreement message to the
designated switch that corresponds to its root port. When the switches 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. The sequence of events is shown in
Figure 8-6
8. Agreement
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 other ports to synchronize.
If the received BPDU is a Rapid PVST+ BPDU with the proposal flag set, the switch 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.
If a designated port receives an inferior BPDU, it immediately replies with its own information.

Detecting Unidirectional Link Failure

The software checks the consistency of the port role and state in the received BPDUs to detect
unidirectional link failures that could cause bridging loops.
Cisco Nexus 5000 Series Switch CLI Software Configuration Guide
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Sequence of Events During Rapid Convergence
4. Agreement 1. Proposal
5. Forward
2. Block
9. Forward
6. Proposal
Root port
Designated port
Figure 8-6.
Edge port
3. Block
11. Forward
7. Proposal 10. Agreement
Chapter 8 Configuring Rapid PVST+
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