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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OSPF Troubleshooting !!!!

Step 1: Verify Basic Connectivity

✅ ping <neighbor IP> → Ensure devices can reach each other.
✅ show interfaces status → Check interface status (UP/Down).
✅ show ip arp → Verify correct MAC resolution.

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Step 2: Check OSPF Neighbor Relationship

✅ show ip ospf neighbor → Check if neighbors are in Full state.
✅ debug ip ospf adj → Identify adjacency issues.

🔴 If neighbors are stuck in INIT or EXSTART:

• Check MTU mismatch → show ip ospf interface
• Check Hello/Dead timer mismatch → show ip ospf interface
• Check Network Type mismatch (Broadcast, P2P, etc.)
• Check Area ID mismatch → show ip ospf interface brief
• Check for authentication issues → show running-config | section ospf

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Step 3: Validate OSPF Route Advertisements

✅ show ip route ospf → Confirm OSPF routes are present.
✅ show ip ospf database → Verify LSAs are exchanged properly.
✅ debug ip ospf lsa → Check if LSAs are being received.

🔴 If missing routes:

• Check if the interface is passive → show ip ospf interface brief
• Check LSA filtering or stub area restrictions
• Check for route summarization issues

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Step 4: Check OSPF Path Selection & Costs

✅ show ip ospf interface → Verify OSPF cost settings.
✅ show ip ospf database router → Inspect LSA details.
✅ show ip route <destination> → Check the chosen OSPF path.

🔴 If unexpected routes appear:

• OSPF cost may be too high/low → Adjust with ip ospf cost <value>
• Check for ECMP issues (Equal-Cost Multipath Routing)

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Step 5: Verify External Route Redistribution (If applicable)

✅ show ip ospf database external → Check Type-5 LSAs.
✅ show running-config | section redistribute → Ensure correct redistribution.
✅ debug ip ospf events → Monitor redistribution behavior.

🔴 If external routes are missing:

• Redistribution misconfigured → redistribute <protocol> subnets
• ACLs or route-maps filtering OSPF routes
• NSSA blocking external routes

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Step 6: Debug & Monitor Logs

✅ debug ip ospf hello → Analyze Hello packet issues.
✅ debug ip ospf packet → Monitor overall OSPF packet exchange.
✅ show logging → Check for error messages.

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