Troubleshooting FCS and RX Errors: What They Mean and How to Fix Them on Your Network Gear

If you’ve ever dug into the interface stats on a router or switch and seen a growing count of FCS errors or RX errors, you’re likely wondering: what do these mean—and should I be worried?

The short answer: yes, you should investigate. These errors can indicate deeper issues in your network that may be causing packet loss, poor performance, or degraded throughput. Sometimes these problems only become apparent when the link is heavily utilized.

Let’s break down what these errors actually are.

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What Are FCS Errors?

FCS stands for Frame Check Sequence. It’s a type of checksum used at the end of Ethernet frames to ensure data integrity.

When a device (like a computer, switch, or router) sends a frame across the network, it calculates an FCS value based on the frame’s contents and appends it to the frame. The receiving device then performs the same calculation. If the result doesn’t match the received FCS, the frame is considered corrupted and is discarded.

Causes of FCS Errors:

• Bad cabling (damaged Ethernet cables, improperly crimped ends)
• Electrical interference (EMI from power sources, fluorescent lights)
• Duplex mismatches (one side set to half-duplex, the other to full)
• Failing network interface cards (NICs)
• Dirty fiber connectors or mismatched optics

Important note: FCS errors are only seen on receive (RX) interfaces, because the device receiving the frame is the one performing the FCS check.

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What Are RX Errors?

RX errors (Receive errors) are a broader category. They include any kind of error that occurs when a switch or router attempts to receive a frame or packet.

FCS errors are actually a subset of RX errors, but RX errors may also include:

• Alignment errors – when frames don’t end on an expected byte boundary
• Runts – frames that are smaller than the minimum Ethernet frame size (usually 64 bytes)
• Giants – frames larger than the max allowable size (often >1518 bytes unless jumbo frames are configured)
• Overruns – when the device’s buffer can’t keep up with incoming traffic
• Symbol errors – typically seen on fiber interfaces, indicating issues at the physical layer

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How to Diagnose & Fix FCS/RX Errors

1. Check cabling
   • Swap out the Ethernet or fiber patch cable
   • Reseat or clean fiber connectors
   • Check for physical damage
2. Verify speed and duplex
   • Mismatched duplex settings are a common cause
3. Replace transceivers or NICs
   • Bad SFPs or network cards can introduce signal corruption
4. Check for interference
   • Keep cables away from sources of EMI like power cables or motors. This is more important with ethernet cables than fiber.
5. Monitor over time
   • Use tools like LibreNMS, Zabbix, or built-in logs to track error counts
     

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Why Do These Errors Matter?

Even a small number of FCS or RX errors can cause:

• Packet retransmissions
• Application performance issues
• VoIP call drops or jitter
• Slow file transfers

In mission-critical networks—like data centers, ISPs, or enterprise backbones—these issues can become costly very quickly.

FCS and RX errors are often the canary in the coal mine for physical-layer issues in your network. Don’t ignore them. They’re your early warning system for bigger problems.

When in doubt, remember: the cleaner the signal, the cleaner the packets. #routethelight

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