Inter-VLAN Routing Router-on-a-Stick

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

📌 What is Inter-VLAN Routing?

Inter-VLAN routing is the process of enabling communication between different VLANs using a Layer 3 device (router or L3 switch).

👉 Simple:

“Different VLANs can talk using a router.”

🧪 Lab Topology (Build in Cisco Packet Tracer)

Devices:

• 1 Switch (2960)
• 1 Router (2911)
• 4 PCs

🔹 VLAN Plan

⚙️ Step 1: Assign IP Addresses

⚙️ Step 2: Create VLANs (Switch)

enable
configure terminal

vlan 10
name HR

vlan 20
name IT

⚙️ Step 3: Assign Access Ports

interface fa0/1
switchport mode access
switchport access vlan 10

interface fa0/2
switchport mode access
switchport access vlan 20

interface fa0/3
switchport mode access
switchport access vlan 10

interface fa0/4
switchport mode access
switchport access vlan 20

⚙️ Step 4: Connect Switch → Router (IMPORTANT)

Use Straight-through cable

Connect:

Switch Fa0/23 → Router G0/0

⚙️ Step 5: Make Switch Port TRUNK

• interface fa0/23
• switchport mode trunk

⚙️ Step 6: Configure Router (CORE STEP 🔥)

enable
configure terminal

interface g0/0
no shutdown

interface g0/0.10
encapsulation dot1Q 10
ip address 192.168.10.254 255.255.255.0

interface g0/0.20
encapsulation dot1Q 20
ip address 192.168.20.254 255.255.255.0

🧪 Step 7: Test Connectivity

Same VLAN

ping 192.168.10.2

✔ Works

🔹 Different VLAN (MAIN TEST)

From PC0:

ping 192.168.20.2

✔ Now it will WORK

Important Note:

Ethernet switches remember MAC addresses (Layer 2) to map devices to physical ports, while routers remember IP addresses (Layer 3) to map devices to logical network addresses. Switches use a temporary CAM table, while routers use DHCP leases to "remember" these assignments.

Key Takeaways:

Switch (MAC): Learns source MAC addresses from incoming frames and associates them with a port. These entries are temporary (typical 300-second aging timer).

Router (IP): Assigns IP addresses via DHCP to specific MAC addresses. These assignments ("leases") can be configured as static/persistent.

Function: Switches enable local network communication (same network), while routers manage traffic between different networks.

🔍 What Happens Internally

• PC0 → sends packet to gateway (router)
• Router receives via VLAN 10 sub-interface
• Router routes to VLAN 20
• Sends back to PC3

👉 “Packet goes:

PC → Switch → Router → Switch → Destination”

👉 “Router performs Layer 3 decision”

⚠️ Common Mistakes

• ❌ Forgetting trunk on switch
• ❌ Missing encapsulation dot1Q
• ❌ Wrong default gateway
• ❌ Router interface shutdown

🧠 Key Concepts

• Router uses sub-interfaces
• Each VLAN = one sub-interface
• Router acts as gateway

🎯 Interview Questions

❓ What is Router-on-a-Stick?

👉 Using one physical router interface with multiple sub-interfaces for VLAN routing

❓ Why trunk is required?

👉 To carry multiple VLANs between switch and router

❓ Why ping was failing earlier?

👉 No Layer 3 routing between VLANs

🧠 Real-World Use

Used in:

• Small office networks
• Labs and training
• Basic enterprise setups