Redundancy makes an industrial network more resilient. But when redundant Ethernet paths are left unmanaged, the same connections designed to protect uptime can bring communication to a halt.
Multiple active paths between switches can create a network loop, allowing Ethernet frames to circulate continuously. As those frames are forwarded and duplicated, they can consume available bandwidth and interfere with communication between controllers, drives and other connected equipment.
Spanning Tree Protocol (STP) addresses this risk by establishing one active route through the network while keeping redundant connections available as alternate paths. The result is controlled redundancy: the network gains a backup route without allowing traffic to loop indefinitely.
Why Can Network Redundancy Become a Problem?
Consider an industrial network connecting a controller to a drive through three switches: Switch A, Switch B, and Switch C.
The controller and drive have two possible communication paths:
- Path 1: Controller → Switch A → Switch C → Drive
- Path 2: Controller → Switch A → Switch B → Switch C → Drive
At first glance, two available routes may seem preferable to one. If both routes are active at the same time, however, the switches do not have a defined way to determine which path traffic should follow.
The physical topology contains a loop. A frame forwarded from one switch can return through the alternate connection and be forwarded again. Instead of reaching its destination and leaving the network, the frame may continue circulating.
What Happens When Ethernet Frames Enter a Loop?
Ethernet frames do not include a built-in mechanism that limits how long they can remain on a switched network. When a loop is present, broadcast and other flooded traffic can be forwarded repeatedly between switches. As traffic circulates, frames may be duplicated and consume an increasing share of the network’s bandwidth. This condition is commonly called a broadcast storm.
The effect can extend beyond the individual frame or device that initiated the traffic. Communication throughout the affected network may slow down or become unavailable, making it difficult for critical industrial devices to exchange information reliably. Redundant connections alone, therefore, do not guarantee resiliency. The network also needs a method for deciding which connections should actively forward traffic.
How Does STP Choose a Single Active Path?
STP allows switches to exchange information about the network topology and establish a loop-free forwarding structure.
One switch is selected as the root bridge, providing a central reference point for path calculations. The switches then evaluate the available routes and determine which ports should participate in the active topology.
Ports along the selected path remain in the forwarding state. A port that would complete a loop is placed in the discarding state. That port remains physically connected but does not forward normal network traffic while the preferred path is available.
In the example network, suppose Switch C is intentionally configured as the root bridge. STP could place the appropriate port on the connection between Switch A and Switch B into the discarding state.
Traffic between the controller and drive would then follow one defined route:
- Controller → Switch A → Switch C → Drive
The physical connection through Switch B still exists, but it does not create a second active path. STP has converted a loop-prone physical topology into a controlled, loop-free logical topology.
What Is the Root Bridge?
The root bridge serves as the reference point STP uses to calculate the network’s active paths. Because its location influences those paths, root-bridge selection can affect how traffic moves through the network.
The root bridge is selected using the switches’ bridge identifiers:
- The switch with the lowest bridge priority is preferred.
- If multiple switches have the same priority, the switch with the lowest MAC address becomes the root bridge.
- A priority of 0 should be used to make your preferred switch the root.
If priorities are left at their default settings, the root may be selected based on MAC address alone. The resulting topology may function, but it may not direct traffic along the route intended by the network designer. For that reason, root-bridge placement should be a deliberate design decision rather than an accidental outcome.
What Happens When the Primary Path Fails?
A port in the discarding state is not necessarily out of service. The connection remains available as part of the network’s redundant physical design. If the active connection between Switch A and Switch C fails, STP can recalculate the topology. The alternate route through Switch B can then transition into service:
- Controller→ Switch A → Switch B → Switch C → Drive
This is the value of controlled redundancy. During normal operation, STP prevents the alternate connection from creating a loop. When the topology changes, that same connection can provide another route through the network.
Why Should the STP Configuration Be Consistent?
Different versions of Spanning Tree Protocol are available. The selected Spanning Tree implementation should be applied consistently across the switching environment. The configuration should also support the intended root bridge, forwarding route and alternate path. Consistency helps ensure that the logical topology reflects the physical network’s design goals rather than leaving critical path decisions to default settings.
Final Takeaway
Redundant Ethernet links can improve availability, but unmanaged redundancy can also create switching loops, broadcast storms and widespread communication problems. Spanning Tree Protocol provides the control that a redundant switched network needs. It establishes a root bridge, creates one loop-free forwarding path and keeps alternate connectivity available when the topology changes. The objective is not simply to add more connections. It is to build controlled redundancy—with a defined active route, an intentional root bridge and a standby path ready when it is needed.
Network & Security Specialist at Kendall Electric
Larry has over 16 years of experience spanning ISP deployments, campus-wide BACnet systems, statewide physical security networks, and manufacturing consulting. He holds a Bachelor’s in Electrical Engineering and a Master’s in Enterprise Architecture from Penn State. Larry is known for his ability to simplify complex networking concepts and his passion for continuous learning. He’s especially focused on emerging trends like micro segmentation and edge computing.