The Frame that got away: Frame Check Sequence (FCS) Failures in Network Switches

Once upon a time, you were a hopeless romantic who decided to send a love letter using a carrier pigeon to deliver it across a bustling city. You carefully wrote your heartfelt message, tied it securely to the pigeon’s leg, and set it on its way. But what if the pigeon got lost, the letter was damaged, or, worst of all, the pigeon forgot the route entirely? In networking terms, the pigeon represents the “frame,” the letter is the “data,” and the city symbolizes the “network.”

Understanding FCS

Before we discuss about FCS we need to know what a frame is. A frame switch is a device used in networking that operates at the Data Link Layer (Layer 2) of the OSI model. It forwards data packets (frames) between devices based on their MAC addresses. Unlike hubs, which broadcast data to all connected devices, switches intelligently direct frames only to the intended recipient, improving network efficiency and reducing unnecessary traffic.

• Preamble: Gets devices ready to send and receive data.
• SFD (Start Frame Delimiter): Marks where the data starts in the frame.
• Destination MAC: Address of the device receiving the data.
• Source MAC: Address of the device sending the data.
• Type/Length: Shows the kind of data or its size.
• Data (Payload): The actual information being sent.
• FCS (Frame Check Sequence): Checks for errors in the data.

A Frame Check Sequence (FCS) is an error-detecting code that is added to the end of a data frame in a communication protocol. This extra data helps ensure data integrity during transmission. ?

Think of the FCS as a security seal for your message. When you send a frame (a unit of data), you attach this seal (a special code) created using a mathematical trick called the cyclic redundancy check (CRC). It’s like a pigeon’s tracking tag that represents the exact contents of the frame.

When the frame arrives at its destination, the switch (the pigeon inspector) checks the tracking tag, recalculates it based on the actual contents of the frame, and verifies if it matches. If the numbers don’t add up, the switch knows the frame has been tampered with or damaged—and it stamps "REJECTED!" on it.

Types of Errors

Here are some errors that might occur as our pigeon takes flight with the royal letter:

1. Input Errors

- This counter will increase when the interface receives a frame with any kind of error, this includes runts, giants, no buffer available, CRC errors, etc.? ?

- These errors occur when a device receives a frame incorrectly. It’s like the pigeon arriving with the letter but with ink smudges all over it.

2. Output Errors

- These happen when frames can’t be sent properly. Imagine the prince writing a heartfelt letter but the pigeon’s wings are too tired to fly. This counter increases when anything goes wrong with the transmission of the packet.??

3. CRC Errors

- Cyclic Redundancy Check (CRC) errors indicate a mismatch in the checksum. It’s like the seal on the letter being broken, suggesting tampering.

4. Runt Frames

?? - These are frames smaller than the minimum allowed size (less than 64 bytes). Picture a letter so short it doesn’t even get to say, “Yours sincerely.”

5. Giant Frames

?? - Frames exceeding the maximum allowed size (1518 bytes, no Jumbo frames). Think of you gave the pigeon a briefcase so heavy to carry it.

6. Collisions

- When two devices transmit simultaneously, causing frame corruption. It’s like two pigeons bumping into each other mid-flight and scattering their letters.

Causes of FCS Failures

Several factors can lead to FCS failures in a network:

1. Physical Layer Issues

- Damaged cables, loose connectors, or electromagnetic interference (EMI) can cause bit errors during transmission.

2. Network Congestion

- Overloaded ports or links can lead to dropped or corrupted frames. Imagine too many pigeons trying to land on the same perch.

3. Hardware Failures

- Malfunctioning network interface cards (NICs), switch ports, or faulty transceivers.

4. Duplex Mismatches

- Mismatched duplex settings between connected devices (e.g., one set to full-duplex and the other to half-duplex) can cause collisions and frame corruption.

5. Software or Firmware Bugs

- Bugs in the switch’s operating system or firmware can lead to improper frame processing.

6. Excessive Network Noise

- Noise from nearby electronic devices or poor grounding can interfere with signal transmission.

Identifying FCS Failures

Switches and routers typically track FCS errors on their interfaces. These errors can be viewed using network management tools or command-line interfaces (CLI). Below are examples of commands used on Cisco switches:

Example 1: Viewing Interface Statistics

In this output, note the 20 CRC errors and 5 frame errors. These indicate FCS failures on the interface.

Steps to Troubleshoot and Mitigate FCS Failures

1. Ensure Physical Layer Integrity

- Damaged?cables and connectors?must?be checked.

- Use certified Ethernet cables that meet?the required specifications (Cat 5e,?Cat 6,?and so on)?for,?say,?gigabit links.

2. Check Duplex and Speed Settings Match

- Duplex and speed settings?should be verified if they match?on both?sides?of a link:

- If there is a mismatch in settings, then fix it using the following commands:

3. Replace Faulty Hardware

- Replace?suspected faulty components,?including?transceivers, cables, or NICs.

4. Update Firmware or Drivers

- Check the manufacturer’s website for any available firmware updates or patches.

- Ensure all connected devices have the latest drivers installed.

5. Monitor and Analyze Traffic

- Use monitoring tools such as Wireshark to capture and analyze packets to get more detail about the errors.

6. Check Environmental Factors

?? - Ensure?that?network?equipment?is?properly grounded.

?? - Reduce?interference by?moving?cables away from high-EMI devices.

Real-World Scenario: Troubleshooting FCS Failures

Scenario

A network?administrator?is?experiencing?a switch interface connected to a server?that has poor performance. The?following?command output?reveals?a?large?number of CRC errors.

Action Plan

Step 1: Problem Identification

• The admin first runs the following command:

• Or the admin can see the interface individually as below

• The admin observes high CRC errors.

Step 2: Inspect Physical Connections

• Admin physically inspects the cables and finds a damaged patch cord.
• Replaces the patch cord with a new Cat 6 cable.

Step 3: Verify Duplex and Speed Settings

• Checks the settings with:

• Finds a mismatch with the server configured to full-duplex. Updates the switch settings:

Step 4: Monitor the Interface

• Monitors the interface after changes:

• Confirms the issue is resolved.

Step 5: Document and Proactively Check Other Interfaces

• Logs the steps taken to resolve the issue.
• Schedules regular checks for other interfaces to preempt future issues.

Outcome: The network's performance improves significantly, and no further errors are observed on the problematic interface.

Conclusion

FCS failures are?one?of the?common?problems?that can?seriously?affect?network performance if not?handled?in time.?Knowing?their causes and?using?systematic troubleshooting?approaches, network administrators?are?able?to?trace?and?eliminate?them?as fast as possible. Regular monitoring and proactive maintenance can further?reduce?the?likelihood?of FCS failures?and?assure?a robust and reliable network.

So next time you debug a network?problem,?think?of?yourself as the prince,?getting?his love letter?to?its destination without?smudge?or?tear,?without?clash of wings. Happy troubleshooting!