Network Troubleshooting by OSI Model

This troubleshooting process guide applies to all Ethernet-based Network devices.

It follows the OSI Model, which are the building blocks of network connectivity. 

Most consumer devices use all 7-layers, but Industrial devices will use as little as 3. 

For the purposes of this guide, we will stop at layer-4.

We start with the bottom of the stack first, as those are the simpler and more likely points of failure.

Layer 1 — Physical

Focus: Cables, connectors, power, physical link

1. Check link lights
   • Verify NIC, switch ports, and intermediate devices show link/activity.
   • No light = no physical connection.

2. Inspect cabling
   • Ensure cables are fully seated.
   • Test with known‑good cable.
   • Look for damage or kinks.

3. Verify power to network devices
   • Switches, routers, PoE injectors should be powered.
   • If PoE device → check if PoE budget is exhausted.

4. Port testing
   • Try another Ethernet port on device (if available)
   • Try a USB-to-Ethernet adapter (if possible)

5. Check for speed/duplex mismatch
   • If the switch forces Gigabit but cable is Cat5 (not 5e or higher), issues occur.

Layer 2 — Data Link

Focus: MAC addressing, ARP, switching

1. Check NIC status
   • Windows PowerShell: Get-NetAdapter

• Confirm if the interface is “Up” 
  1. “Down” = Physically connected to something but not being allowed on the network (i.e. Port has Static IP incompatible with the Gateway)

2. Check Switch Settings
   • Is there a switch connected between the devices?
   • If so, is it Managed?
     1. Ensure no MAC/IP Address filtering, blocked ports, etc.
     2. Ensure VLANs are configured correctly
   • If not, an unmanaged switch will always broadcast all packets, no filters.
     1. But it can also cause network storms if a ring/loop is formed

3. Detect duplex mismatch
   • Slow or intermittent? Duplex and/or Link Speed mismatch is possible.
   • Ensure both devices are set for Auto Negotiation 
     1. Note: Emerson PACSystems Ethernet is not user configurable, and is always Auto Negotiation

4. Test ARP
   • Windows PowerShell or Command Prompt: arp -a

• Look for device

Layer 3 — Network

Focus: IP addresses, routing, subnets

1. Verify IP configuration
   • Windows PowerShell or Command Prompt: ipconfig

• Check: 
  • Correct IP?
  • Correct subnet mask?
  • Correct gateway?
  • BOTH devices must be on the same Subnet and have a similar IP

2. Check local connectivity
   • Ping other devices
   • Do other network devices respond, but you can’t reach one of them?

3. Check routing table
   • Windows Command Prompt: route print
   • Ensure default route exists.

Layer 4 — Transport

Focus: TCP/UDP ports, sessions

1. Test common ports
   • PowerShell: Test-NetConnection -Port <port>

2. Check for packet loss
   • Command Prompt: pathping <ipaddress>

3. Verify if firewall is blocking ports
   • Windows Firewall, or physical security appliance

4. Check MTU issues
   • Jumbo frames mismatches cause silent drops.
   • Command Prompt: ping <ipaddress> -f -l <size>