CCNA Spanning Tree Protocol BPDU Guard Configuration
BPDU
BPDU stands for bridge protocol data unit. BPDUs are data messages that are exchanged across the switches within an extended LAN that uses a spanning tree protocol topology. BPDU packets contain information on ports, addresses, priorities and costs and ensure that the data ends up where it was intended to go. BPDU messages are exchanged across bridges to detect loops in a network topology. The loops are then removed by shutting down selected bridge interfaces and placing redundant switch ports in a backup, or blocked, state.
To configure BPDU Guard, you have to first g enable Portfast, and then, you should be able to enable BPDU Guard. Also, you can configure globally, but it is not recommended.
Understanding How PortFast BPDU Guard Works
To prevent loops from occurring in a network, the PortFast mode is supported only on nontrunking access ports because these ports typically do not transmit or receive BPDUs. The most secure implementation of PortFast is to enable it only on ports that connect end stations to switches. Because PortFast can be enabled on nontrunking ports connecting two switches, spanning tree loops can occur because BPDUs are still being transmitted and received on those ports.
PortFast BPDU guard prevents loops by moving a nontrunking port into an errdisable state when a BPDU is received on that port. When you enable BPDU guard on the switch, spanning tree shuts down PortFast-configured interfaces that receive BPDUs instead of putting them into the spanning tree blocking state. In a valid configuration, PortFast-configured interfaces do not receive BPDUs. If a PortFast-configured interface receives a BPDU, an invalid configuration exists. BPDU guard provides a secure response to invalid configurations because the administrator must manually put the interface back in service.
