By default, devices in different VLANs cannot communicate because they exist in separate broadcast domains. To enable traffic flow between them, we need Inter-VLAN Routing.
Layer 3 Requirement
Inter-VLAN routing requires a Layer 3 device, such as a router or a Multilayer Switch, to move packets between subnets.
Implementation Methods
1. Router-on-a-Stick (ROAS)
A single physical link connects a router to a switch. The router interface is divided into logical sub-interfaces, each tagged with a VLAN ID (802.1Q).
2. Layer 3 Switch (SVI)
A Switch Virtual Interface (SVI) is created for each VLAN. This is the fastest method as routing happens at hardware speeds directly on the switch.
Comparison Table
| Feature | Router-on-a-Stick | Layer 3 Switch (SVI) |
|---|---|---|
| Hardware | Router + L2 Switch | Multilayer Switch |
| Performance | Shared Link (Limited) | Wire-speed (Very Fast) |
| Configuration | Sub-interfaces | VLAN Interfaces |
Common Error
On a Layer 3 switch, Inter-VLAN routing will NOT work unless you enable it globally with the `ip routing` command!
CLI Quick Look (ROAS)
Router(config)# interface g0/0.10 Router(config-subif)# encapsulation dot1Q 10 Router(config-subif)# ip address 192.168.10.1 255.255.255.0