STP. Part 1. STP (802.1D-1998)
STP. Part 2. RSTP (802.1w)
STP. Part 3. PVST+
STP. Part 4. MSTP
STP. Part 5. Bridge Assurance
Bridge Assurance is a STP extension that protects L2 network from any unidirectional link event caused by physical cable failure or adjacent switch control plane failure.
Without Bridge Assurance, BPDU are sent only downstream (from Root down the Tree). With Bridge Assurance enabled, on every point-to-point trunk link the switch expects to receive BPDUs for every VLAN that is active on this trunk link (in case of RSTP). If BPDU is not received for a configured VLAN, this VLAN is blocked on this port in terms of STP. You can think of this as an automatic pruning for your trunks. If the blocked port begins receiving BPDUs again, it is removed from bridge assurance blocking state, and goes through normal Rapid-PVST transition.
The Bridge Assurance feature is useful in regular spanning-tree topologies that do not use vPC. If a switch in a looped spanning-tree topology stops sending BPDUs because the control plane is not functioning correctly, then Bridge Assurance suspends the ports to prevent a loop.
By default, Bridge Assurance is enabled globally, so if a port is configured with spanning-tree port type network command, Bridge Assurance is enabled on that port.
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