How To Troubleshoot: InterVLAN Routing.
As I have mentioned earlier on other troubleshooting page(s), one of the commonest mistake administrators make during networking is made during configuration stage, either on the router, switch or logical subnet addressing.
On this page(s), we’ll look at the challenges; common issues and troubleshooting methods related with configuring multiple VLANs on a network.
If you suspect that there is a problem with a switch configuration, use the show interface (interface-id) switchport command for verification.
The show running-config and the show interface (interface-id) switchport commands are useful Cisco IOS troubleshooting tools for identifying VLAN assignment and port configuration issues.
When using the traditional routing model for inter-VLAN routing, ensure that the switch ports that connect to the router interfaces are configured on the correct VLANs. If the switch ports are not configured or assigned correctly to VLANs, network devices configured on the VLANs will not receive or connect to the router interface, which in turns hinder traffic to other VLANs on the network.
Using the Topology above, PC2 and router R1 interface F0/1 are configured to share the same subnet. However, the switch port F0/3 that connects to router R1 interface F0/1 has not been configured and remains in the default VLAN. Because router R1 is on a different VLAN than PC2, they are unable to communicate.
To solve this problem, use the switchport access vlan 20 interface configuration command on switch port F0/3 on switch SW1. When the switch port is configured or assigned to the correct VLAN, PC2 can communicate with router R1 interface F0/1, which will then enable access to other VLANs connected to router.