What is the typical troubleshooting order when diagnosing issues across network layers?

What is the typical troubleshooting order when diagnosing issues across network layers?

• A. L1/L2 (link, errors, light) → L2 (VLAN/LACP) → L3 (routes, ACLs, MTU, DNS).
• B. L3 checks first, then L2, then L1.
• C. L1 checks, then L3, then L2.
• D. L2 checks first, then L1, then L3.

Answer: (A)

Start with the physical and data-link layers. First, verify the actual connection: cables plugged in, port status, correct speed/duplex, and any link light indicators, plus any visible errors. If the link isn’t healthy, traffic can’t move regardless of higher-layer settings, so fixing L1 is the priority. If the physical and link layers look good, move to the data-link layer. Check that VLANs are correctly configured and present on the right switches, that trunks carry the necessary VLANs, and that any link aggregation (LACP) is negotiated properly. Misconfigurations here can prevent devices on the same network from communicating even when the physical link is up. Only after L1 and L2 checks are clear should you examine the network layer. At this stage, look at routing decisions and routes, as well as ACLs that might block traffic, MTU settings that could cause fragmentation, and DNS for name resolution. These factors determine reachability across networks and to external destinations, but they rely on a solid physical and data-link foundation. This bottom-up approach ensures you address the root cause first and prevents chasing issues that can’t be resolved until the lower layers are functioning.

Start with the physical and data-link layers. First, verify the actual connection: cables plugged in, port status, correct speed/duplex, and any link light indicators, plus any visible errors. If the link isn’t healthy, traffic can’t move regardless of higher-layer settings, so fixing L1 is the priority.

If the physical and link layers look good, move to the data-link layer. Check that VLANs are correctly configured and present on the right switches, that trunks carry the necessary VLANs, and that any link aggregation (LACP) is negotiated properly. Misconfigurations here can prevent devices on the same network from communicating even when the physical link is up.

Only after L1 and L2 checks are clear should you examine the network layer. At this stage, look at routing decisions and routes, as well as ACLs that might block traffic, MTU settings that could cause fragmentation, and DNS for name resolution. These factors determine reachability across networks and to external destinations, but they rely on a solid physical and data-link foundation.

This bottom-up approach ensures you address the root cause first and prevents chasing issues that can’t be resolved until the lower layers are functioning.