Router On a Stick

1. Introduction

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.

2. Concept of Subinterfaces

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).

3. Router on a Stick Configuration

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

We start with the physical interface Gi0/1 and add labels to create the subinterfaces:

• Gi0/1.10 → VLAN 10

• Gi0/1.20 → VLAN 20

The subinterface number does not have to match the VLAN ID, but in practice we usually make them the same. This makes the configuration much easier to read and maintain.

Next, we bind each subinterface to a VLAN using 802.1Q encapsulation, and we assign an IP address that will serve as the default gateway for that VLAN.

Here is the configuration:

R1(config)# interface gigabitEthernet 0/1.10
R1(config-subif)# encapsulation dot1Q 10
R1(config-subif)# ip address 192.168.1.1 255.255.255.0
R1(config-subif)# no shutdown
R1(config-subif)# exit

R1(config)# interface gigabitEthernet 0/1.20
R1(config-subif)# encapsulation dot1Q 20
R1(config-subif)# ip address 192.168.2.1 255.255.255.0
R1(config-subif)# no shutdown
R1(config-subif)# end

Subinterface Checklist

To configure a subinterface, we always follow the same 4 steps:

1. Create the subinterface (Gi0/1.x)

2. Apply encapsulation (encapsulation dot1Q )

3. Set the IP address (gateway for the VLAN)

4. Enable it (no shutdown)

Verification

Now that the subinterfaces are configured, let’s check that everything is working as expected.

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
GigabitEthernet0/1.10      192.168.1.1   YES manual   up up
GigabitEthernet0/1.20      192.168.2.1   YES manual   up up
GigabitEthernet0/2         unassigned    YES unset    administratively down down

• The physical interface Gi0/1 has no IP address. This is normal in Router on a Stick, because only the subinterfaces handle IP routing.

• Both subinterfaces Gi0/1.10 and Gi0/1.20 are up/up with their correct IP addresses.

This confirms that our subinterfaces are active and ready.

Next, let’s look at the routing table:

R1# show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route

Gateway of last resort is not set

  192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
C   192.168.1.0/24 is directly connected, GigabitEthernet0/1.10
L   192.168.1.1/32 is directly connected, GigabitEthernet0/1.10
  192.168.2.0/24 is variably subnetted, 2 subnets, 2 masks
C   192.168.2.0/24 is directly connected, GigabitEthernet0/1.20
L   192.168.2.1/32 is directly connected, GigabitEthernet0/1.20

Here we see that R1 has learned both networks:

• VLAN 10 → 192.168.1.0/24 via Gi0/1.10

• VLAN 20 → 192.168.2.0/24 via Gi0/1.20

This means the router now knows how to route traffic between the two VLANs.

4. InterVLAN Routing Process

Let’s walk through InterVLAN routing step by step, together.

On PC1 (VLAN 10), we use the default gateway 192.168.1.1 (R1 subinterface Gi0/1.10):

C:\> ipconfig

FastEthernet0 Connection:(default port)

   Connection-specific DNS Suffix..: 
   Link-local IPv6 Address.........: FE80::2D0:D3FF:FE42:BD92
   IPv6 Address....................: ::
   IPv4 Address....................: 192.168.1.10
   Subnet Mask.....................: 255.255.255.0
  Default Gateway.................: ::
                                     192.168.1.1

On the Server (VLAN 20), it’s the same idea: the default gateway is 192.168.2.1 (R1 subinterface Gi0/1.20).

Now, from PC1 we ping the server 192.168.2.10:

C:\> ping 192.168.2.10

Pinging 192.168.2.10 with 32 bytes of data:

Reply from 192.168.2.10: bytes=32 time<1ms TTL=127
Reply from 192.168.2.10: bytes=32 time<1ms TTL=127
Reply from 192.168.2.10: bytes=32 time<1ms TTL=127
Reply from 192.168.2.10: bytes=32 time<1ms TTL=127

Ping statistics for 192.168.2.10:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
    Minimum = 0ms, Maximum = 0ms, Average = 0ms

The ping is successful, but it’s important to understand why.

PC1 sends a packet to 192.168.2.10. Since the destination IP address is outside its own network, the packet is forwarded to the default gateway 192.168.1.1, which is the router subinterface Gi0/1.10.

First, the switch receives the packet on F0/1 (assigned to VLAN 10) and forwards the frame towards the default gateway over the trunk link Gi0/1. On this trunk, the switch encapsulates the frame with the VLAN ID 10.

Figure 5 — PC1 to Default Gateway (R1 Subinterface)

The router R1 receives the packet on its physical interface Gi0/1.
Since the frame is tagged with VLAN 10, subinterface Gi0/1.10 processes it.

R1 then makes a routing decision: the destination IP is 192.168.2.10, which belongs to the 192.168.2.0/24 network. That network is connected to subinterface Gi0/1.20, so the packet is routed there.

The router rewrites the Ethernet header, changes the VLAN tag from 10 to 20, and sends the packet back on the trunk.

Figure 6 — InterVLAN Routing Process

The switch receives this frame (now tagged VLAN 20) and forwards it to the server on F0/2.

As you can see, the InterVLAN Routing process combines:

• Layer 2 technologies → VLANs and 802.1Q trunking

• Layer 3 decisions → Router subinterfaces forwarding between VLANs

If we zoom out, the entire flow looks like this:

Figure 7 – Packet entire Flow with Router on a Stick

PC1 → SW1 (tag VLAN 10) → R1 Gi0/1.10 → R1 routes → R1 Gi0/1.20 (tag VLAN 20) → SW1 → Server

Router on a Stick Limitation

Now that you’ve seen the process, it’s important to highlight one major limitation.

All subinterfaces are configured on a single physical interface (Gi0/1).
This means all inter-VLAN traffic must pass through the same physical link.

Figure 8 – Router on a Stick bottleneck

If we add, for example, 50 VLANs, this single interface quickly becomes a bottleneck, because bandwidth is limited to that one link.

To overcome this limitation, modern designs use Layer 3 switches with SVIs, allowing inter-VLAN routing to happen directly in the switch fabric. We will cover this method in the next course.

5. Conclusion

Router on a Stick is a simple way to enable communication between VLANs using subinterfaces.

• 802.1Q subinterfaces on one router interface

• Routing between VLANs with a single link

• Limitation: all traffic on the same interface → bottleneck

• Better option: Layer 3 switches with SVIs

This method works for small or lab networks, but it does not scale well.
In the next lesson, we will see how SVIs on Layer 3 switches solve these limits.