When it comes to spanning tree topologies, there are different ways that ports can behave. We’ve already discussed point-to-point ports and shared ports between switches. But there’s a third type of port known as an edge port, which is where edge devices connect. Edge devices include PCs, printers, routers, and anything that isn’t connecting switches together.
Why is this important? Spanning tree is designed to prevent loops, which occur in devices that forward layer 2 frames. Hubs and switches can cause loops, but edge devices do not forward frames, so they don’t contribute to loop formation. Therefore, there’s no need to worry about blocking ports or configuring port states when edge devices are connected.
However, in the past, it took a significant amount of time for a port connected to an edge device to come online. This wasted valuable time. To address this issue, Cisco introduced a feature called PortFast. Even though it was part of the vendor-neutral Rapid Spanning Tree Protocol (RSTP), it became a built-in feature of the standard and is commonly referred to as the edge port.
Configuring a port as an edge port or enabling PortFast is done through the interface configuration on Cisco switches. By entering the “spanning tree portfast” command, we indicate that certain ports are only for edge devices. This allows the ports to bypass some of the spanning tree procedures, improving their performance. It’s worth noting that other vendors may refer to this feature as an edge port.
When PortFast or edge port is enabled, the interface is listed as an edge port in the spanning tree details. It immediately enters the forwarding mode, bypassing the discarding, learning, and forwarding states. Additionally, if this port goes up or down, it will not trigger a topology change notification (TCN) to other switches.
But what happens if someone accidentally or maliciously plugs a switch into an edge port? By default, this could introduce a loop into the network, which is a critical issue. To address this, there’s another feature called BPDU Guard. Although it may not be part of the exam, it’s essential to mention it briefly.
When an edge port is configured with PortFast and BPDU Guard, the network is protected from loops even if a switch is connected to the edge port. If a switch is connected, it will start sending Bridge Protocol Data Units (BPDUs). The original switch will recognize these BPDUs and realize that a switch is connected. Consequently, the port will lose its edge port status and become a regular port. This ensures the network remains secure and free from loops.
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FAQs
Q: What are edge devices?
A: Edge devices are devices such as PCs, printers, routers, and any other devices that are not connecting switches together.
Q: Why is PortFast important?
A: PortFast allows ports connected to edge devices to come online immediately, avoiding unnecessary waiting time.
Q: What is BPDU Guard?
A: BPDU Guard is a feature that protects the network from loops by disabling a port if it receives Bridge Protocol Data Units (BPDUs), indicating a switch is connected to an edge port.
Conclusion
In conclusion, Cisco’s Rapid Spanning Tree Protocol (RSTP) includes the concept of edge ports or PortFast. This feature allows ports connected to edge devices to bypass some spanning tree procedures, improving their performance. Additionally, by enabling BPDU Guard, the network remains protected from loops, even if a switch is mistakenly connected to an edge port. By understanding and configuring these features correctly, network engineers can ensure a more efficient and secure network infrastructure.
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