Core
Inter-VLAN Routing
Inter-VLAN routing allows hosts in different VLANs to communicate, implemented via router-on-a-stick subinterfaces or Layer 3 switching with SVIs.
What you need to know
- Router-on-a-stick uses one physical interface divided into subinterfaces, each assigned a VLAN and IP via encapsulation dot1Q.
- Each subinterface must match the VLAN's IP subnet and use encapsulation dot1Q <vlan-id> before an IP address can be applied.
- The subinterface handling the native VLAN typically omits the native keyword unless the native VLAN itself is being routed with tagging disabled.
- Layer 3 switches use SVIs (interface vlan X) instead of subinterfaces, avoiding the single physical link bottleneck.
- ip routing must be enabled globally on a multilayer switch for SVIs to route between VLANs.
- A switch port connecting to a router for router-on-a-stick must be configured as a trunk, not access.
- SVIs require the associated VLAN to exist and have at least one active access/trunk port to stay up/up in some IOS versions.
Configuration commands
| Command | Mode | Purpose |
|---|---|---|
| interface gigabitethernet 0/0.10 | config | Create subinterface for VLAN 10 on router-on-a-stick. |
| encapsulation dot1Q 10 | config-if | Bind the subinterface to VLAN 10 tagging. |
| ip address 192.168.10.1 255.255.255.0 | config-if | Assign the gateway address for VLAN 10 hosts. |
| interface gigabitethernet 0/0.99 | config | Create subinterface for the native VLAN. |
| encapsulation dot1Q 99 native | config-if | Mark this subinterface as carrying the untagged native VLAN. |
| ip routing | config | Enable IP routing globally on a multilayer switch. |
| interface vlan 10 | config | Create an SVI for VLAN 10 on a multilayer switch. |
| no switchport | config-if | Convert a physical switch port into a routed Layer 3 port. |
Verify it
- show ip interface brief
- show ip route
- show interfaces gi0/0.10
- show vlan brief
- show interfaces trunk
Common mistakes
- Forgetting to configure the switch-side port as a trunk for router-on-a-stick.
- Mismatching the VLAN number in encapsulation dot1Q with the actual VLAN on the switch.
- Not enabling ip routing on a multilayer switch, so SVIs never route traffic.
- Assigning an IP address to the physical interface as well as subinterfaces, causing conflicts.
- Leaving the SVI or associated VLAN with no active ports, causing the SVI to stay down.
Practise Inter-VLAN Routing in a real CLI
Generated labs, graded against the simulated network state.