How to Configure Router-on-a-Stick (Inter-VLAN Routing)
To configure router-on-a-stick: trunk the switch port toward the router, then create a subinterface per VLAN on the router with encapsulation dot1q and an IP. Each subinterface becomes that VLAN's gateway.
Router-on-a-stick routes between VLANs over a single link. See the router-on-a-stick lab.
Step 1: Trunk the switch port to the router
On the switch:interface gi0/1switchport mode trunk | The link to the router carries all VLANs |
Step 2: Bring up the router physical interface
On the router:interface gi0/0no shutdown | No IP on the physical interface itself — it just carries the trunk |
Step 3: Create a subinterface per VLAN
interface gi0/0.10encapsulation dot1q 10ip address 192.168.10.1 255.255.255.0 | VLAN 10 gateway — repeat for each VLAN (.20, .30…) |
Verification
show ip interface brief shows the subinterfaces up with their IPs. Set each PC's gateway to its subinterface IP — devices in different VLANs can now ping each other, proving inter-VLAN routing works.
Frequently asked questions
How do I configure router-on-a-stick?
Trunk the switch port to the router, then on the router create a subinterface per VLAN with encapsulation dot1q [vlan] and an IP address — each becomes that VLAN's gateway.
Why does the physical router interface have no IP?
It only carries the trunk; each dot1q subinterface holds the IP and acts as its VLAN's gateway.
What does encapsulation dot1q 10 do?
It tells the subinterface to tag and accept frames for VLAN 10 on the trunk link.
Is router-on-a-stick used in real networks?
In small networks yes; larger ones use Layer 3 switches with SVIs for higher performance, but the concept is identical.
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