CCNA Spanning Tree Protocol Configure PortFast

What is PortFast?

Portfast is a Cisco Propietary feature that was originally developed to overcome an issue when a PC was trying to obtain a DHCP address and going past the DHCP process before the port had gone into a forwarding state. The portfast feature brought the port into a forwarding state as soon as it came up enabling the PC to obtain a DHCP address. The feature was then standardised and is now called the Edge port.

Understanding How PortFast BPDU Guard Works

To prevent loops from occurring in a network, this 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 this mode 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.

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