Cisco Nexus 7000 Series Configuration Manual page 195

Configuring STP Extensions Using Cisco NX-OS
Bridge Assurance
Bridge Assurance is enabled by default and can only be disabled globally. Also, Bridge Assurance can be
enabled only on spanning tree network ports that are point-to-point links. Finally, both ends of the link must
have Bridge Assurance enabled. If the device on one side of the link has Bridge Assurance enabled and the
device on the other side either does not support Bridge Assurance or does not have this feature enabled, the
connecting port is blocked.
With Bridge Assurance enabled, BPDUs are sent out on all operational network ports, including alternate and
backup ports, for each hello time period. If the port does not receive a BPDU for a specified period, the port
moves into the blocking state and is not used in the root port calculation. Once that port receives a BPDU, it
resumes the normal spanning tree transitions.
This figure shows a normal STP topology.
Figure 17: Network with Normal STP Topology
This figure demonstrates a potential network problem when the device fails and you are not running Bridge
Assurance.
Figure 18: Network Problem without Running Bridge Assurance
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