Router-on-a-Stick (ROAS): Complete Guide

A Router-on-a-Stick (ROAS) is an Inter-VLAN routing method where a single physical router interface routes traffic between multiple Virtual Local Area Networks (VLANs) across a single physical 802.1Q trunk link.

Because VLANs logically isolate Layer 2 broadcast domains, hosts in different VLANs cannot communicate without a Layer 3 routing engine. ROAS solves this by using virtual sub-interfaces on the router, where each sub-interface acts as the default gateway for an assigned VLAN.

How ROAS Operates

1. An endpoint in VLAN 10 transmits a frame directed to an endpoint in VLAN 20, using its local default gateway IP (the router's sub-interface).
2. The Layer 2 switch tags the frame with an IEEE 802.1Q tag (VLAN ID 10) and pushes it across the trunk link to the router.
3. The router receives the tagged frame on sub-interface .10, de-encapsulates the Layer 2 header, inspects the destination IP, and matches it to the directly connected network of VLAN 20.
4. The router rebuilds a new Layer 2 frame tagged for VLAN ID 20 and sends it back down the exact same physical link to the switch.
5. The switch strips the 802.1Q tag and delivers the untagged frame to the destination access port.

ROAS Network Topology

                      +-----------------------------+
                      |        Cisco Router         |
                      |   Physical Port: G0/0/0     |
                      |  .10 Gateway: 192.168.10.1  |
                      |  .20 Gateway: 192.168.20.1  |
                      +--------------+--------------+
                                     |
                                     | Single Physical Link
                                     | (802.1Q Trunk)
                                     v
                      +--------------+--------------+
                      |     Layer 2 Switch          |
                      |     Uplink: Gig 0/1 (Trunk) |
                      +-------+-------------+-------+
                              |             |
               [Fa0/1] Access |             | [Fa0/2] Access
                    (VLAN 10) |             | (VLAN 20)
                              v             v
                           +----+         +----+
                           |PC 1|         |PC 2|
                           +----+         +----+
                        192.168.10.10  192.168.20.10

Cisco CLI Configuration Walkthrough

1. Switch Trunk Port Configuration

Switch(config)# interface gigabitethernet 0/1
Switch(config-if)# switchport mode trunk
Switch(config-if)# switchport trunk allowed vlan 10,20
Switch(config-if)# no shutdown
Switch(config-if)# exit

2. Router Physical and Sub-Interface Configuration

! Bring up the main physical interface without an IP address
Router(config)# interface gigabitethernet 0/0/0
Router(config-if)# no shutdown
Router(config-if)# exit

! Configure Sub-interface for VLAN 10
Router(config)# interface gigabitethernet 0/0/0.10
Router(config-subif)# encapsulation dot1Q 10
Router(config-subif)# ip address 192.168.10.1 255.255.255.0
Router(config-subif)# exit

! Configure Sub-interface for VLAN 20
Router(config)# interface gigabitethernet 0/0/0.20
Router(config-subif)# encapsulation dot1Q 20
Router(config-subif)# ip address 192.168.20.1 255.255.255.0
Router(config-subif)# exit

ROAS vs. Multilayer Switching (SVIs)

| Feature | Router-on-a-Stick (ROAS) | Multilayer Switch (SVI) | |---|---|---| | Hardware Requirement | Layer 2 Switch + External Router | Layer 3 Switch (Multilayer) | | Gateway Location | Router Sub-interfaces (G0/0/0.10) | Internal SVIs (interface Vlan 10) | | Link Bandwidth | Shared uplink bottleneck (half-duplex path) | Internal backplane (wire-speed ASIC routing) | | Best Use Case | Small networks, lab environments, branch offices | Enterprise campus LANs and core networks |

Essential Verification Commands

Note:

• show ip interface brief — Confirms sub-interfaces are created and in an up / up state.
• show ip route — Verifies that directly connected subnets (C) are present in the routing table.
• show vlans — On Cisco routers, displays active 802.1Q VLAN sub-interfaces, encapsulation types, and packet counters.

🛠️ Hands-On Cisco Packet Tracer Lab Files

Download the complete Router-on-a-Stick lab topology and official configuration guide: