Inter-VLAN routing allows devices in different VLANs to talk to each other. This is very useful for managing traffic and keeping networks organized, but sometimes problems can happen. Below are some common issues and simple solutions.
1. Wrong VLAN Setup
Problem
The VLANs are not set up correctly on the switch. Devices may be in the wrong VLAN or the VLAN may not even exist.
Solution
Check that every VLAN is created on the switch and that each port is assigned to the right VLAN. For access ports, use commands like switchport mode access and switchport access VLAN X. For trunk ports, use switchport mode trunk and make sure the right VLANs are allowed.
2. Trunk Port Misconfiguration
Problem
The trunk link between the switch and the router (or between switches) is not set up correctly. This can block VLAN traffic.
Solution
Make sure the trunk port allows all VLANs that are needed with the switchport trunk allowed VLAN command. Also confirm that the trunk uses the correct encapsulation type (usually 802.1Q) with the switchport trunk encapsulation dot1q command.
3. Router-on-a-Stick Errors
Problem
The router sub-interfaces are not configured properly. Each VLAN should have its own sub-interface on the router, but sometimes these are missing or incorrect.
Solution
Check that each router sub-interface has the right VLAN tag and IP address. Use commands like:
interface GigabitEthernet0/0.10
encapsulation dot1Q 10
ip address 192.168.10.1
4. Wrong IP Addresses
Problem
Devices in different VLANs have IP addresses that do not match their subnet. This stops them from communicating.
Solution
Confirm that every device has an IP address from the correct subnet. Also, check that the subnet mask is correct so the router knows how to send traffic.
5. Missing Routing Information
Problem
The router does not know how to reach all the VLANs. Without routes, traffic will not move between networks.
Solution
Make sure the router has routes for each VLAN. Usually, directly connected routes are enough, but sometimes you may need to add static routes or use a dynamic routing protocol like OSPF or EIGRP.
6. ACLs or Firewalls Blocking Traffic
Problem
Access Control Lists (ACLs) or firewall rules may be stopping traffic between VLANs.
Solution
Review any ACLs on the router or switches. Remove or update rules that block the traffic you need. Make sure only unwanted traffic is blocked, not everything.
7. Native VLAN Mismatch
Problem
The native VLAN on one end of a trunk link is different from the other end. This causes traffic to be sent to the wrong VLAN.
Solution
Check that the native VLAN is the same on both devices. You can set it with the switchport trunk native vlan command.
8. Spanning Tree Protocol (STP) Issues
Problem
A bad STP configuration can block important ports or even cause loops in the network. This will affect inter-VLAN routing.
Solution
Look at the STP settings and make sure there are no loops. You can change STP priorities to control which switch becomes the root bridge and optimize the path for traffic.
Conclusion
Inter-VLAN routing lets devices in different VLANs talk to each other. This makes the network more organized and easier to manage. But if it is not set up the right way, problems can happen, like wrong VLANs, trunk port mistakes, bad IP addresses, missing routes, or blocked traffic.
To fix these problems, you should check each part step by step. Make sure VLANs are set up, trunk ports work, router interfaces are correct, IP addresses are right, and routing rules allow traffic. If everything is checked carefully, inter-VLAN routing will work well and make the network stronger and more reliable.