intermediate v1 • Updated 7 hours ago

Inter-VLAN Routing (Router-on-a-Stick & SVI)

Inter-VLAN routing is the process of forwarding network traffic between different VLANs using a Layer 3 routing device or Layer 3 switch.

Remember This Key Rule

VLANs isolate at Layer 2; Inter-VLAN routing connects them at Layer 3 using Router-on-a-Stick or Layer 3 Switch SVIs.

In Simple Terms

Because VLANs completely isolate Layer 2 broadcast domains, traffic cannot cross from VLAN 10 to VLAN 20 on its own. You need a Layer 3 device (either a router using subinterfaces or a Layer 3 multilayer switch using Switch Virtual Interfaces - SVIs) to route packets between the VLANs.

Visual Diagram & Flow

Router (Gi0/0.10: .10.1 | Gi0/0.20: .20.1)

▲ 802.1Q Trunk Link ▼

Switch (VLAN 10 Ports | VLAN 20 Ports)

Why It Matters in Real Networks

Inter-VLAN routing is how enterprise networks maintain segmentation while still allowing authorized users in one department to access shared file servers or the Internet.

How It Works (Technical Breakdown)

Two primary methods: 1. **Router-on-a-Stick (ROAS)**: A single physical router interface connects to a switch trunk. The router interface is split into virtual subinterfaces (e.g. `Gi0/0.10` for VLAN 10, `Gi0/0.20` for VLAN 20), each with an 802.1Q encapsulation tag and its own IP gateway address. 2. **Multilayer (L3) Switch SVI**: The switch handles routing internally at wire-speed hardware (ASIC) level using virtual interface IPs (`interface vlan 10`, `interface vlan 20`), avoiding router link bottlenecks.

Real-World Analogy

Two secure adjacent rooms with no connecting door. To pass a document, you must hand it out the window to a security guard in the hallway (the router), who inspects it and passes it through the other window.

Concrete Example

Cisco Router-on-a-Stick configuration: `interface GigabitEthernet0/0.10` ` encapsulation dot1Q 10` ` ip address 192.168.10.1 255.255.255.0` `interface GigabitEthernet0/0.20` ` encapsulation dot1Q 20` ` ip address 192.168.20.1 255.255.255.0`

Common Engineering Mistakes & Pitfalls

Forgetting to configure the switch port connected to the router as an 802.1Q trunk, causing subinterfaces to fail silently.

Prerequisites

IPv4 Addressing & Classless Architecture (CIDR) Recommended VLANs (Virtual Local Area Networks) & 802.1Q Recommended

Next & Related Concepts

Static Routing & Default Routes Unlocks Next →

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Version	Change Summary	Author	Timestamp
v1 (Current)	Active production content	system	Sun, Aug 23, 2026 11:09 AM