Which of the following technologies should be used to prevent a switching loop if a switch is connected to a port
configured for PortFast?

A.
RSTP

B.
BPDU Guard

C.
Root Guard

D.
PVST

Explanation:
BPDU Guard prevents switching loops in the case of a switch being connected to a PortFast interface. PortFast
is used for ports that connect to host systems, such as workstations and printers, and allows the port to
immediately enter a forwarding state. This bypasses the normal 30-second delay that Spanning Tree Protocol
would normally use to determine if a switch has been connected to the port. Implementing BPDU Guard will
disable the port if a switch is connected and a BPDU is received.
Rapid Spanning Tree Protocol (RSTP) is incorrect because this is an enhanced Spanning Tree standard that
operates on the Data Link layer of the OSI model. RSTP was not designed to protect PortFast ports. PortFast
and BPDU Guard are supported by RSTP, but they not required or configured by default.
Root Guard is incorrect because it is used to protect the root bridge placement in the Spanning Tree, not to
protect PortFast ports.
Per-VLAN Spanning Tree (PVST) is incorrect because this is an implementation of Spanning Tree (the default
protocol for Cisco switches), and was not designed to protect PortFast ports. PortFast and BPDU Guard aresupported by RSTP, but are not required, and must be configured manually.
Objective:
LAN Switching Fundamentals
Sub-Objective:
Configure, verify, and troubleshoot STP-related optional features

Cisco > Support > Configuring Spanning Tree PortFast, BPDU Guard, BPDU Filter, UplinkFast, and LoopGuard
> Understanding How PortFast Works
CCNA Routing and Switching Complete Study Guide: Exam 100-105, Exam 200-105, Exam 200-125, 2nd
Edition, Chapter 2: LAN Switching Technologies – Configure, verify, and troubleshoot STP protocols