27.12.1 PortFast

(1) Overview

PortFast is functionality for ports for which a terminal is connected and loops are known in advance not to occur. PortFast is not subject to Spanning Tree topology calculations, allowing communication immediately after link-up.

(2) BPDU reception when PortFast is applied

PortFast is set for ports for which no BPDUs are expected to be received, but when a BPDU is received on a port for which PortFast is set, a switch might exist ahead, meaning that a loop is possible. Therefore, PortFast functionality stops, and operation starts as a normal port subject to Spanning Tree operations including topology calculations and BPDU sending and reception.

After operation starts as a port subject to Spanning Tree operation, PortFast functionality is enabled again by links being brought up or down.

Use this in combination with the BPDU filter functionality to prevent PortFast functionality from stopping when a BPDU is received.

(3) Send BPDU when PortFast is applied

Because Spanning Tree Protocols cannot run on ports for which PortFast is set, BPDUs are not sent.

However, to detect whether ports with PortFast set are mistakenly connected, BPDUs are sent for only 10 frames immediately after communication becomes possible due to PortFast functionality.

(4) BPDU guard

Functionality applied to PortFast includes the BPDU guard functionality. On ports for which the BPDU guard functionality is applied, when a BPDU is received, the port becomes inactive, instead of running as a Spanning Tree target port.

Ports put in the inactive status can be released using the activate command, to link up again with PortFast ports with the BPDU guard functionality applied, and resume communication.