Troubleshooting Packet Drops in a Switch

When packets are getting dropped on a switch, it can result from various issues across different layers. Here's a structured way to troubleshoot packet drops on a switch:

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🔍 Troubleshooting Packet Drops in a Switch

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1. Check Interface-Level Counters

Use the following commands (based on switch OS):

• Cisco IOS:

  show interfaces [interface-id]
  show interfaces counters errors
  show interfaces status err-disabled
  

• Look for:

  • Input/output errors

  • CRC errors (Layer 1 issue)

  • Collisions (Half-duplex problems)

  • Drops (buffers exhausted)

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2. Verify Port Configuration

• Speed/Duplex mismatch:

  show interfaces [interface-id] status
  show running-config interface [interface-id]
  

  • Ensure both sides of a link have the same speed/duplex settings.

  • Autonegotiation issues can cause excessive drops.

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3. Check for Congestion or Buffer Overflows

• Causes:

  • High traffic load on uplinks

  • Microbursts

  • Insufficient buffer allocation

• Commands:

  show platform hardware capacity [interface/buffer stats]
  show queueing interface [interface-id]      (on some platforms)
  

• Solution:

  • Use QoS to prioritize important traffic.

  • Increase buffer sizes (if configurable).

  • Load-balance traffic over multiple links (EtherChannel).

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4. Look for Broadcast Storms or Loops

• Symptoms:

  • High CPU usage

  • Drops on multiple interfaces

  • MAC flaps

• Commands:

  show mac address-table
  show spanning-tree
  show processes cpu sorted
  

• Fixes:

  • Enable/verify Spanning Tree Protocol (STP)

  • Enable Storm Control

  • Check for loopback cables or misconnected devices

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5. Inspect QoS Policies

• Misconfigured QoS can lead to packet drops in input/output queues.

• Commands:

  show policy-map interface [interface-id]
  

• Check for:

  • Drop counters under QoS class-maps

  • Policing or shaping issues

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6. CPU or Control Plane Congestion

• Some traffic gets punted to CPU (e.g., ARP, STP BPDUs). Excessive control traffic can overwhelm the switch CPU.

• Commands:

  show processes cpu
  show platform cpu packet statistics
  

• Fixes:

  • Apply CoPP (Control Plane Policing)

  • Offload traffic processing if possible

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7. Hardware Failures or Bugs

• Bad interfaces, line cards, or known software bugs can also cause unexplained drops.

• Steps:

  • Check logs: show logging

  • Check for hardware errors: show environment, show module

  • Search vendor bug database (e.g., Cisco Bug Toolkit)

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🔧 Summary of Common Causes

Issue Type	Common Symptoms	Quick Fix
Duplex mismatch	CRC errors, collisions	Match speed/duplex manually
Congestion	Interface drops	Use QoS, upgrade link
STP/loops	CPU spike, flooding	Enable STP, storm control
QoS misconfig	Output queue drops	Tune policies, verify classes
Hardware/bugs	Unexplained drops	RMA or firmware upgrade

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