Inter-VLAN routing issues can come in many forms, especially considering that multiple devices are involved (switches, routers, etc.) in the process. By following a proper troubleshooting methodology, you should be able to isolate the problem to a particular device and then map it to a specific feature that has been misconfigured.
From a connectivity standpoint, some of the things that need to be checked include:
- Verfiying that the end-stations are connected in the proper switch ports
- Verifying that the proper switch ports are connected in the proper router ports (if a router is used for inter-VLAN routing)
- Verifying that each of the ports involved in this process carry the correct VLANs
- The ports that connect the end-stations are usually access ports allocated to a particular VLAN
- The ports connecting the switch to the router are usually trunk ports
After confirming that the connectivity between the devices is correct, the next logical step is investigating Layer 2 configuration, starting with the configured encapsulation method on the trunk ports, which is usually 802.1Q, the preferred method. Next, make sure that the same encapsulation is configured on both ends of the trunk link.
Some of the commands that can be used to verify the encapsulation types are as follows:
- show interface trunk
- show interface <number> switchport
Here is an example output:
Cat-3550-1#show interfaces trunk
Port Mode Encapsulation Status Native vlan
Fa0/1 on 802.1q trunking 1
Fa0/2 on 802.1q trunking 1
Port Vlans allowed on trunk
Fa0/1 1,10,20,30,40,50
Fa0/2 1-99,201-4094
Another important detail that is offered by the show interface trunk command is the trunk status. This confirms whether the trunk is formed or not and it has to be checked at both ends of the link. If the interface is not in “trunking” mode, one of the most important things that has to be verified is the mode of operation (on, auto, etc.) to see whether it will allow a trunking state to form with the other end of the link.
The native VLAN is another important element that you should verify on the trunk ports. Misconfigured native VLANs can lead to a lack of functionality or security issues. The native VLAN should match at both ends of the trunk links.
If after verifying the Layer 2 verification tasks the inter-vlan issue is still not resolved, you can proceed to verifying Layer 3 configuration. Depending upon the Layer 3 device used to ensure the actual inter-VLAN routing, this can be configured/verified on one of the following devices:
- Multilayer switch
- Router – physical interfaces
- Router – subinterfaces
On the Layer 3 device, you should verify that the correct subnet is assigned to each interface (or SVI), and you should also verify the routing protocol, if needed. Usually, a different subnet is assigned to each VLAN so you should make sure that you don’t misconfigure the interfaces. In order to verify this you can use the show interface command for the specific physical interface, subinterface, or SVI.