Which command is used to enable PortFast on a Cisco switch interface?
Correct Answer: A
Detailed Explanation: PortFast is a Cisco feature designed to improve network convergence times on access ports—those ports connected directly to end devices such as workstations, printers, or IP phones. Under normal circumstances, when a switch port comes up, it goes through a series of Spanning Tree Protocol (STP) states (blocking, listening, learning, and finally forwarding) to ensure that no loops are created in the network. This process, while essential for loop prevention, can take up to 30 seconds, which is often unnecessary for ports that connect to end devices rather than to other switches. The command "spanning-tree portfast" is used in interface configuration mode to enable this feature. When PortFast is enabled, the port immediately transitions to the forwarding state, bypassing the usual STP listening and learning states. This rapid transition reduces the time a device must wait before it can start communicating on the network, which is especially beneficial during boot-up or when a device is connected or reconnected. How PortFast Improves Network Convergence Immediate Forwarding: Normally, when a port is brought up, it must wait through the STP convergence process. For end devices, this delay can slow down network access and user productivity. With PortFast enabled, the port immediately starts forwarding traffic, minimizing connection delays. Ideal for Access Ports: PortFast should only be used on ports connected to end-user devices. Since these devices do not introduce switching loops (unlike a connection between switches), bypassing the STP process is safe. Using PortFast on a port connected to another switch can result in network loops, so best practice dictates that it be limited to edge ports. Enhanced User Experience: Reducing the delay for end devices is critical in environments where rapid network access is required. For example, in VoIP deployments, minimizing delay is essential to avoid disruptions in call quality. Similarly, in environments where users frequently disconnect and reconnect (such as in hot-desking scenarios), PortFast helps ensure that devices obtain network connectivity almost immediately. Impact on Network Stability: Although PortFast improves convergence times, it must be applied judiciously. Misconfiguration—such as enabling PortFast on a trunk or inter-switch link—can inadvertently create network loops. To mitigate this risk, Cisco offers additional features (like BPDU Guard) that can automatically shut down a PortFast-enabled interface if a BPDU (Bridge Protocol Data Unit) is detected. This added security measure prevents misconfigured ports from causing loops. Practical Implementation: To enable PortFast on a specific interface, an administrator would access the interface configuration mode and issue the command: Router(config)# interface FastEthernet0/1 Router(config-if)# spanning-tree portfast This configuration immediately instructs the switch to bypass the STP states for that interface. Troubleshooting and Verification: After enabling PortFast, administrators can verify its operation using commands like: show spanning-tree interface FastEthernet0/1 detail This output will indicate that the port is in the forwarding state without the usual STP transition delay. Conclusion The "spanning-tree portfast" command (Option A) is the correct and standard way to enable PortFast on a Cisco switch interface. It is a critical feature for reducing convergence time on access ports, thereby enhancing user experience and overall network performance—provided it is applied only to appropriate ports. Mastery of PortFast and its proper implementation is an important skill for network professionals preparing for the CCNA exam and managing real-world networks.
This CCNA practice question helps students prepare for Cisco networking certification exams by testing knowledge of network fundamentals, routing, switching, and network security concepts.