What is a Network Loop?

Quick Definition: A network loop is an undesired condition in computer network traffic where data packets circulate endlessly within a network due to redundant connections between devices, potentially leading to congestion and network collapse.

What is a Network Loop?

Imagine a chain of routers passing traffic through a network. If a device later in the chain directs traffic back to a device earlier in the chain, the traffic could keep following that chain repeatedly, getting sent back to the beginning and looping forever.

But network loop prevention isn't just making sure no devices later in the chain ever send traffic back to an earlier router – clearly, that would be impossible. Most networks are so large and complex, and connections between devices so intricate, that potential loops exist everywhere. Redundancy, high availability, and efficient data flow are worth the risk, but the chance for an accidental loop increases with every layer of added complexity.

Fortunately, special protocols like the Spanning Tree Protocol and other loop avoidance methods can prevent them. These methods aren't simple, but if you master them, you're one step closer to earning your CCNA.

And a network administrator with that certification is a valuable addition to any network. Plus, you can say you know how to prevent two of the coolest-sounding problems in networking: a cascading network failure caused by network loops and broadcast storms.

Defining Network Loops and Their Variations

A network loop is defined as a situation where data circulates endlessly, causing congestion and inefficiency in communication. A network loop is caused by misconfigurations or multiple paths in a network. They don't just prevent the network traffic from reaching its intended destination but can sap processing resources and drag the entire network down with it.

Imagine you're on a cross-country road trip, and one of the steps in your direction says, "Go back to the place you started." No matter how far away from home you were, you'd probably suspect something was wrong with your directions. Unfortunately, data packets aren't as smart as you.

All they do is follow directions; if the directions loop back on themselves, the packets won't make a fuss. It's up to network administrators to understand and recognize network loops and loopback processes.

Although loops are especially detrimental in multicast networks, they can still occur in unicast networks. There are different types of network loops, depending on the network layer, network type, and the links involved. Learning those layers, types, and links can be challenging, but courses on associate-level networking skills can help you understand the intricacies. 

Layer 2 network loops are often related to a misconfiguration of the Spanning Tree Protocol, which is designed to identify and prevent redundant paths. Broadcast storms can occur on Layer 2 as well when a network device sends a broadcast message that gets continually rebroadcast by other devices. MAC address tables are instrumental to Layer 2 forwarding, and incorrect MAC table entries can result in incorrectly forwarded packets and loops.

A common cause of Layer 3 network loops is routing loops, which tend to happen when there are inconsistencies in routing tables. Those inconsistencies can be due to human error or an issue with dynamic routing protocols like OSPF or EIGRP.

Network topology and metric values at Layer 3 can also lead to unstable path selection. Suboptimal routing decisions can create a vicious cycle of routing errors, and lead to network loops at Layer 3.

Even VLANs and Power over Ethernet (PoE) networks can create network loops if there are issues with mismatched configurations or power negotiation.

How Do Network Loops Impact Your Network?

Network loops cause a range of bad outcomes for a network, all of which impact the overall functionality and stability of the network. Common network issues caused by loops include: 

• Congestion is the most obvious impact of network loops. The constant retransmission of packets consumes bandwidth and can cause delays for valid traffic. 

• Broadcast storms can happen when unnecessary traffic keeps getting broadcast, overwhelming switches and devices. 

• Latency is a common side effect of network loops, as packets take longer to get where they belong and slow down high-latency apps like voice and video communication.

• Packet loss becomes more prevalent when network loops increase the chances of packets getting dropped or discarded as the looped traffic competes for resources.

• General instability and service outages can create a range of potential issues for a network suffering network loops.

It's often the job of an associate network administrator to recognize the symptoms of a network loop and identify their causes. It takes deliberate training and practice to master the technologies related to handling and preventing network loops.

How to Prevent Network Loops 

Network loops aren't inevitable, and their worst consequences don't have to be, either. Trained and experienced network administrators can identify network loops and mitigate their effects quickly using the correct tools and protocols. 

Spanning Tree Protocol (STP) and Network Loop Prevention

Spanning Tree Protocol (STP) is the most common method of preventing loops in Ethernet networks. STP's function is to identify and block redundant paths and ensure only one active path exists between any two network devices. 

STP works by mapping the network and identifying switches and their paths, according to many different factors. Then, an algorithm calculates the most efficient paths through the network and assigns each port on each switch a specific role and state.

A loop-free topology is established by a process that includes, among other steps, identifying and blocking redundant links. Then, STP continuously monitors the network topology and reacts to changes by recalculating path costs and adapting the entire structure.

Covering the technical components and configurations of STP any further is beyond the scope of this explainer. Still, CBT Nuggets's online CCNA course covers STP and explains its implementation in depth.

Network Loop Prevention Through Other Means

Methods other than STP exist for preventing network loops, but generally speaking, they should be implemented alongside and in addition to STP, not instead of it:

• Some network devices have loop prevention mechanisms like loop guard, root guard, and Bridge Protocol Data Unit (BPDU).

• Port security mechanisms that control the number of MAC addresses allowed on a port can help prevent malicious loops.

• VLANs that logically segment a network can help reduce loops since each VLAN operates independently, and loops would be confined within their boundaries.

• Regular network monitoring tools can detect and identify loop-related issues and allow network administrators to take proactive steps.

• Good network design can't be overstated in network loop prevention. Systems administrators who are trained and educated on modern design techniques and loop prevention methods are the best defense against network loop prevention.

Final Thoughts on Preventing Network Loops 

Network loops are a major concern for networks of basically any size. Broadcast storms can degrade network performance significantly and disrupt normal communication between devices. Network loops aren't always catastrophic, sometimes they're hiding in-between seemingly harmless devices and configurations.

Hidden network loops might not immediately manifest as broadcast storms, which is why they can be tricky to detect. Minor loops and misconfigurations can cause intermittent and subtle issues like increased latency or occasional packet loss.

Learning to prevent network loops proactively is crucial to any network administrator's job. If you want a job maintaining network stability, the CCNA certification can prove to employers you understand not just network loops but also routing protocols like OSPF and EIGRP, WAN, VLAN, and more. Sign up for CBT Nuggets to get unlimited access to CCNA training.