Router-on-a-Stick Inter-VLAN Routing

Router-on-a-Stick Scenario

In the previous topic, three different ways to create inter-VLAN routing were listed, and legacy inter-VLAN routing was detailed. This topic details how to configure router-on-a-stick inter-VLAN routing. You can see in the figure that the router is not in the center of the topology but instead, appears to be on a stick near the border, hence the name.

In the figure, the R1 GigabitEthernet 0/0/1 interface is connected to the S1 FastEthernet 0/5 port. The S1 FastEthernet 0/1 port is connected to the S2 FastEthernet 0/1 port. These are trunk links that are required to forward traffic within and between VLANs.

To route between VLANs, the R1 GigabitEthernet 0/0/1 interface is logically divided into three subinterfaces, as shown in the table. The table also shows the three VLANs that will be configured on the switches.

Router R1 Subinterfaces

 
Subinterface VLAN IP Address
G0/0/1.10 10 192.168.10.1/24
G0/0/1.20 20 192.168.20.1/24
G0/0/1.99 99 192.168.99.1/24

Assume that R1, S1, and S2 have initial basic configurations. Currently, PC1 and PC2 cannot ping each other because they are on separate networks. Only S1 and S2 can ping each other, but they but are unreachable by PC1 or PC2 because they are also on different networks.

To enable devices to ping each other, the switches must be configured with VLANs and trunking, and the router must be configured for inter-VLAN routing.

S1 VLAN and Trunking Configuration

Complete the following steps to configure S1 with VLANs and trunking:

Step 1. Create and name the VLANs.

Step 2. Create the management interface.

Step 3. Configure access ports.

Step 4. Configure trunking ports.

The physical network topology shows two PCs, two switches, and a router. PC1 has IP address 192.168.10.10/24. PC2 has IP address 192.168.20.10/24. 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 on switchport F0/1. Switch S1 is connected to router R1 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

 

Inter-VLAN Routing using Layer 3 Switches