Router On a Stick

Router on a Stick (ROAS) is one of the three methods of Inter-VLAN Routing that we introduced earlier in the InterVLAN Routing course.

To quickly recap, the three approaches are:

• Legacy Inter-VLAN Routing – using multiple physical router interfaces (rarely used today).

• Router-on-a-Stick (ROAS) – using subinterfaces on a single router interface.

• Layer 3 Switch (SVIs) – the modern and scalable solution.

In this lesson, we will focus on Router on a Stick, which allows a network administrator to route packets between multiple VLANs using a single router interface divided into subinterfaces.

This is the reference topology we’ll use throughout the course:

Figure 1 – Router on a Stick Topology

• PC1 in VLAN 10 (192.168.1.0/24)

• Server in VLAN 20 (192.168.2.0/24)

By configuring Router on a Stick, we enable communication between these two VLANs.
The first concept you need to understand for this method is the subinterface.

A subinterface is a virtual interface created under a single physical router interface.
Each subinterface is associated with a specific VLAN ID using 802.1Q encapsulation.

Figure 2 – Subinterfaces on R1

In our example, the physical interface Gi0/1 is divided into two subinterfaces:

• Gi0/1.10 → VLAN 10

• Gi0/1.20 → VLAN 20

Each VLAN gets its own subinterface, which serves as the default gateway for all devices inside that VLAN.

Because we use one physical interface to support multiple VLANs through subinterfaces, this design is called Router on a Stick (ROAS).

Figure 3 – Router on a Stick Topology

Step 1 — Switch: Define VLANs & Assign Access Ports

First, we need to create the VLANs for the Client (VLAN 10) and the Server (VLAN 20).

SW1# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
SW1(config)# vlan 10
SW1(config-vlan)# name client
SW1(config-vlan)# exit

SW1(config)# vlan 20
SW1(config-vlan)# name server
SW1(config-vlan)# exit

Once the VLANs are created, we assign the access ports to the right VLANs.

SW1(config)# interface fastEthernet 0/1
SW1(config-if)# switchport mode access
SW1(config-if)# switchport access vlan 10
SW1(config-if)# exit


SW1(config)# interface fastEthernet 0/2
SW1(config-if)# switchport mode access
SW1(config-if)# switchport access vlan 20
SW1(config-if)# exit

Step 2 — Switch: Configure the 802.1Q Trunk to the Router

Now, we need to prepare the connection between the switch and the router.
Since the link must carry multiple VLANs, we convert interface Gi0/1 into a trunk port.

SW1(config)# interface gigabitEthernet 0/1
SW1(config-if)# no shutdown
SW1(config-if)# switchport mode trunk

SW1(config-if)# switchport trunk ?
allowed Set allowed VLAN characteristics when interface is in trunking mode
native Set trunking native characteristics when interface is in trunking mode

SW1(config-if)# switchport trunk allowed vlan 10,20
SW1(config-if)# end

Here, we allow only VLAN 10 and VLAN 20 on the trunk to restrict unnecessary traffic.

Note: In real networks, it’s good practice to use a dedicated native VLAN that is not assigned to users. For our lab, leaving the default is fine.

Step 3 — Router: Bring Up the Physical Interface

Next, we move to the router.
We start by enabling the physical interface Gi0/1 , this is the “stick” where we will create subinterfaces.

R1# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)# interface gigabitEthernet 0/1
R1(config)# no shutdown

Let’s quickly verify that the interface is up:

R1# show ip int brief
Interface                IP-Address   OK?  Method   Status           Protocol
GigabitEthernet0/0       unassigned   YES  unset    administratively down down
GigabitEthernet0/1       unassigned   YES  unset    up               up
GigabitEthernet0/2       unassigned   YES  unset    administratively down down

Step 4 — Router: Create Subinterfaces (One per VLAN)

Now that our physical interface is ready, we can create one subinterface for each VLAN.
This step is quite straightforward, so let’s walk through it together.

Figure 4 – Router on a Stick Subinterface Configuration