Initial Assessment Steps

• Verify physical connectivity first - cables, ports, and power status
• Check link lights on both ends of connection (solid green = good link)
• Test with known working cable to eliminate cable issues
• Confirm correct cable type (straight-through vs crossover for older devices)

Layer-by-Layer Troubleshooting Approach

Physical Layer (Layer 1)

• Check cable integrity using cable tester or swap with known good cable
• Verify port status: show interfaces for up/down and line protocol status
• Look for error counters: CRC errors, collisions, or late collisions
• Confirm duplex and speed settings match on both ends (auto-negotiation failures common)

Data Link Layer (Layer 2)

• Verify VLAN configuration matches on both ends
• Check trunk/access port configuration: show interfaces switchport
• Confirm MAC address table entries: show mac address-table
• Validate Spanning Tree Protocol isn’t blocking ports: show spanning-tree

Network Layer (Layer 3)

• Verify IP addressing and subnet configuration
• Test local connectivity with ping to default gateway
• Check routing table: show ip route for destination networks
• Confirm NAT/PAT configuration if crossing network boundaries

Common No Connection Scenarios

Key Commands for Diagnosis

• show interfaces - Interface status and statistics
• show ip interface brief - Quick interface overview with IP addresses
• ping <destination> - Test Layer 3 connectivity
• traceroute <destination> - Identify where packets stop
• show arp - Verify ARP table entries for local devices
• show cdp neighbors - Confirm directly connected devices (Cisco Discovery Protocol)

Vocabulary

• Line Protocol: Layer 2 status indicating whether the interface can send/receive frames
• Duplex Mismatch: One end configured for full-duplex while other uses half-duplex (causes collisions and poor performance)
• Administrative Distance (AD): Router’s trustworthiness rating for routing information sources
• ARP (Address Resolution Protocol): Maps IP addresses to MAC addresses for local network communication

Interface Status Combinations

• Up/Up: Interface is functioning normally at both physical and data link layers
• Down/Down: Physical layer problem - no connectivity detected
• Up/Down: Physical connection exists but data link layer has issues
• Administratively Down/Down: Interface manually disabled with shutdown command

Notes

Practical Tips

• Always start troubleshooting at the physical layer - 80% of network issues are physical
• Use no shutdown command if interface shows “administratively down”
• Golden Rule: If you can ping the default gateway, the problem is likely beyond your local network
• Document interface error counters before and after changes to measure improvement
• For VLAN issues, verify both access VLAN assignment and trunk allowed VLANs

Common Gotchas

• Auto-negotiation can fail between different vendor equipment
• Spanning Tree can take up to 50 seconds to transition port to forwarding state
• Static routes need administrative distance configured if overriding dynamic routing
• Switch ports default to access mode, router subinterfaces need manual VLAN configuration
• Remember: show run shows configuration, show commands show operational status

Escalation Criteria

• Move to next layer only after confirming current layer is functional
• If multiple users affected, suspect infrastructure (switches, routers, ISP)
• Single user issues typically indicate end-device configuration or local connectivity problems