Introduction
A network loop occurs when redundant connections between switches cause data packets to endlessly circulate, leading to broadcast storms, high CPU usage, and network congestion. If left unresolved, network loops can disrupt communication, slow down network performance, and even crash switches or routers.
This guide will help you detect and fix network loops using Spanning Tree Protocol (STP), switch configuration adjustments, and loop prevention techniques.
What Causes a Network Loop?
A network loop can occur due to:
✅ Redundant Cable Connections – Multiple physical links between switches.
✅ Disabled or Misconfigured Spanning Tree Protocol (STP) – STP prevents loops by blocking redundant paths, but if it’s disabled or misconfigured, loops can form.
✅ Faulty Switch or Bridge Behavior – A misbehaving switch can flood the network with traffic.
✅ Incorrect VLAN Configuration – Overlapping VLANs can create loops in multi-switch environments.
✅ Misconfigured Link Aggregation (LAG/Port Channel) – Improper bundling of multiple links can result in a loop.
Now, let’s go through step-by-step troubleshooting to detect and eliminate network loops.
Step 1: Detecting a Network Loop
Common Symptoms of a Network Loop:
🔹 High CPU usage on switches and routers
🔹 Intermittent or slow network connectivity
🔹 Broadcast storms flooding the network
🔹 Devices unable to communicate or dropping from the network
How to Identify a Network Loop:
1️⃣ Check Network Utilization:
Run:
python
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show processes cpu sorted
- If CPU usage is abnormally high, a loop might be present.
2️⃣ Monitor Broadcast Traffic on the Network:
- Use a packet capture tool like Wireshark to identify excessive broadcast/multicast traffic.
3️⃣ Check the MAC Address Table for Flapping Entries:
Run on a switch:
sql
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show mac address-table dynamic
- If the same MAC address appears on multiple interfaces frequently, a loop is likely.
4️⃣ Check STP Status:
Run:
sql
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show spanning-tree
- If STP is disabled, loops can occur.
If you detect a loop, move to Step 2 to resolve it.
Step 2: Disconnect Suspected Looping Links
A quick way to stop a network loop is to disconnect redundant connections between switches or access points.
🔹 Identify links between switches that might be creating a loop.
🔹 Unplug one cable at a time while monitoring network behavior.
🔹 Once the network stabilizes, the last disconnected cable was likely causing the loop.
If disconnecting cables resolves the issue, move to Step 3 to prevent future loops.
Step 3: Enable and Verify Spanning Tree Protocol (STP)
STP (Spanning Tree Protocol) is designed to prevent network loops by blocking redundant paths.
How to Check if STP is Enabled:
🔹 Run:
pgsql
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show spanning-tree summary
🔹 If STP is disabled, enable it:
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spanning-tree mode rapid-pvst
How to Set a Root Bridge (Best Practice for STP):
On the core switch, configure STP priority to ensure it becomes the root bridge:
sql
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spanning-tree vlan 1 root primary
spanning-tree vlan 10 root primary
This ensures all switches in the network follow a structured topology.
If STP settings are correct but loops persist, move to Step 4.
Step 4: Verify PortFast and BPDU Guard Configurations
PortFast allows edge ports to skip the STP listening and learning states, but enabling it on switch-to-switch links can cause loops.
Check if PortFast is Misconfigured:
🔹 Run:
csharp
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show spanning-tree interface <interface_name> detail
🔹 If PortFast is enabled on an inter-switch link, disable it:
perl
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no spanning-tree portfast
Enable BPDU Guard to Prevent Loops on Edge Ports:
BPDU Guard shuts down a port if it receives an STP BPDU (loop prevention mechanism).
kotlin
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interface GigabitEthernet0/2
spanning-tree bpduguard enable
Save the changes and restart the switch.
If loops still occur, proceed to Step 5.
Step 5: Check Link Aggregation (LAG) & VLAN Misconfigurations
If using EtherChannel, LACP, or PAGP, incorrect configurations can create a loop.
Check if LAG/Port Channel is Correctly Configured:
🔹 Run:
pgsql
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show etherchannel summary
🔹 If a port is in an independent state, reconfigure the Port Channel:
kotlin
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interface Port-channel1
switchport mode trunk
switchport trunk allowed vlan all
Check VLAN Overlapping Issues:
🔹 Run:
sql
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show vlan brief
🔹 Ensure VLANs are correctly assigned across switches to avoid VLAN misconfigurations causing loops.
Step 6: Monitor Network Traffic for Recurring Issues
After applying fixes, monitor the network to ensure the loop does not return.
Use Network Monitoring Tools:
🔹 Wireshark – Capture and analyze loop-causing packets.
🔹 SolarWinds Network Analyzer – Detects STP topology changes and high broadcast traffic.
🔹 PRTG Network Monitor – Alerts on unusual bandwidth spikes due to network loops.
If everything functions normally, your network loop issue is resolved! 🎉
Step 7: Implement Best Practices to Prevent Future Loops
✅ Always enable Spanning Tree Protocol (STP) on all switches.
✅ Use BPDU Guard on all access ports to prevent accidental loops.
✅ Implement EtherChannel/LACP for redundant connections instead of leaving multiple active links.
✅ Properly segment VLANs and avoid overlapping VLANs across switches.
✅ Regularly audit network topology to prevent redundant links from being created.
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