VLAN Mismatch
- ✅ Step 1: Can they ping each other?
✔️ Yes → No mismatch. Done.
❌ No → Continue.
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✅ Step 2: Check IP addresses & subnet masks
- Are IPs in the same subnet based on their masks? ✔️ Yes → Continue. ❌ No → Fix subnet mask. | ▼ ✅ Step 3: Are both devices physically plugged in to the same switch? ✔️ Yes → Continue. ❌ No → Check routing between switches. | ▼ ✅ Step 4: Check VLAN assignments
- Are the switch ports in the same VLAN? ✔️ Yes → Check firewall or ACL ❌ No → Fix VLAN mismatch
| Issue | What It Means | What You’ll See |
|---|---|---|
| 🔸 VLAN not allowed on trunk | Switch A tags VLAN 20, but Switch B doesn’t allow it | Devices can’t talk, DHCP fails |
| 🔸 Trunk port misconfigured as access | Tags stripped or not passed | Connectivity fails entirely |
| 🔸 Native VLAN mismatch | One side expects untagged VLAN 1, other expects VLAN 99 | Intermittent weird issues, duplicate traffic, security risk |
| SSH commands |
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show interfaces trunk -
show vlan brief -
show running-config interface Gi0/1