Troubleshoot Inter-VLAN Routing

Common Inter-VLAN Issues

By now, you know that when you configure and verify, you must also be able to troubleshoot. This topic discusses some common network problems associated with inter-VLAN routing.

There are a number of reasons why an inter-VAN configuration may not work. All are related to connectivity issues. First, check the physical layer to resolve any issues where a cable might be connected to the wrong port. If the connections are correct, then use the list in the table for other common reasons why inter-VLAN connectivity may fail.

 
Issue Type How to Fix How to Verify
Missing VLANs
  • Create (or re-create) the VLAN if it does not exist.
  • Ensure host port is assigned to the correct VLAN.
show vlan [brief]
show interfaces switchport
ping
Switch Trunk Port Issues
  • Ensure trunks are configured correctly.
  • Ensure port is a trunk port and enabled.
show interfaces trunk
show running-config
Switch Access Port Issues
  • Assign correct VLAN to access port.
  • Ensure port is an access port and enabled.
  • Host is incorrectly configured in the wrong subnet.
show interfaces switchport
show running-config interface
ipconfig
Router Configuration Issues
  • Router subinterface IPv4 address is incorrectly configured.
  • Router subinterface is assigned to the VLAN ID.
show ip interface brief
show interfaces

Troubleshoot Inter-VLAN Routing Scenario

Examples of some of these inter-VLAN routing problems will now be covered in more detail.

This topology will be used for all of these issues.

The physical network topology shows two PCs, two switches, and a router. PC1 has IP address 192.168.10.10/24, a default gateway of 192.168.10.1 and is in VLAN 10. PC2 has IP address 192.168.20.10/24, a default gateway of 192.168.20.1 and is in VLAN 20. PC1 connects to Switch S1 on switch port F0/6. PC2 connects to Switch S2 on switch port F0/18. Switch S1 and Switch S2 are interconnected to each other over a trunk link on switchport F0/1. Switch S1 is connected to router R1 over a trunk link on switch port F0/5 which connects to the G0/0/1 interfaces on R1. The management IP address on S1 is 192.168.99.2/24. The management IP address on S1 is 192.168.99.3/24