There are several common switch misconfigurations that can arise when configuring routing between multiple VLANs.
When using the legacy routing model for inter-VLAN routing, ensure that the switch ports that connect to the router interfaces are configured with the correct VLANs. If a switch port is not configured for the correct VLAN, devices configured on that VLAN cannot connect to the router interface; therefore, those devices are unable to send data to the other VLANs.
As shown in the Figure 1 topology, PC1 and router R1 interface G0/0 are configured to be on the same logical subnet, as indicated by their IP address assignment. However, the switch port F0/4 that connects to router R1 interface G0/0 has not been configured and remains in the default VLAN. Because router R1 is on a different VLAN than PC1, they are unable to communicate.
To correct this problem, execute the switchport access vlan 10 interface configuration mode command on switch port F0/4 on switch S1. When the switch port is configured for the correct VLAN, PC1 can communicate with router R1 interface G0/0, which allows it to access the other VLANs connected to router R1.
The Figure 2 topology shows the router-on-a-stick routing model. However, interface F0/5 on switch S1 is not configured as a trunk and is left in the default VLAN for the port. As a result, the router is unable to route between VLANs because each of its configured subinterfaces is unable to send or receive VLAN-tagged traffic.
To correct this problem, issue the switchport mode trunk interface configuration mode command on switch port F0/5 on S1. This converts the interface to a trunk port, allowing a trunk to be established between R1 and S1. When the trunk is successfully established, devices connected to each of the VLANs are able to communicate with the subinterface assigned to their VLAN, thus enabling inter-VLAN routing.
The Figure 3 topology shows the trunk link between S1 and S2 is down. Because there is no redundant connection or path between the devices, all devices connected to S2 are unable to reach router R1. As a result, all devices connected to S2 are unable to route to other VLANs through R1.
To reduce the risk of a failed inter-switch link disrupting inter-VLAN routing, redundant links and alternate paths should be accounted for within the network design.