Router on a Stick Configuration in Cisco Packet Tracer

In any network, a VLAN is used to segment the network. Users in a particular VLAN will not have access to resources in other VLANs.
For host devices in one VLAN to be able to communicate with hosts on another VLAN, a router will be needed to forward the traffic from one VLAN to another. This is because each VLAN has its own broadcast domain.
In a network with two or three VLANs, it is possible to use individual router interfaces to handle traffic from each VLAN in the network. However, as the number of VLANs increases, this becomes inefficient as the router has limited physical interfaces. One solution to this problem is to replace the router with a Layer 3 switch and use the Layer 3 switch to implement inter-VLAN routing. However, this is expensive, especially when the network has not grown very big.

The most cost-effective way to achieve inter-VLAN routing is by implementing Router on a stick configuration. Here, instead of using each router’s physical interfaces to carry traffic from a specific VLAN, we create router sub-interfaces and then use each sub-interface to carry traffic from one VLAN to another.
In this post, I will guide you through the process of implementing inter-vlan routing using a router in a stick configuration using a sample network topology.

Network Topology

The network topology we will be making use of in this post is shown below. As you can see, it consists of two VLANs that are connected to the router using one physical interface through the switch. In these demonstrations, we will create two sub-interfaces on the router so that hosts in each VLAN can use one sub-interface to forward traffic to the other VLAN.

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Router on a stick configuration

Router on a Stick Configuration: Steps

Here are steps to implement Router on a Stick Configuration using the network topology shown above.

Step 1: Create the Router sub-interfaces

Firstly, Bring the g0/0/0 interface of router0 up.

R0(config)#interface g0/0/0
R0(config-if)#no shutdown

Then configure sub-interfaces g0/0/0.10, enable dot1q encapsulation, and assign it to VLAN10. Put the IP on interface g0/0/0.10.

R0(config-if)#interface g0/0/0.10
R0(config-subif)#encapsulation dot1Q 10
R0(config-subif)#ip address

Configure sub-interfaces g0/0/0.20, enable dot1q encapsulation, and assign it to VLAN20. Put the IP on interface g0/0/0.20.

R0(config)#interface g0/0/0.20
R0(config-subif)#encapsulation dot1Q 20
R0(config-subif)#ip address

Step 2: Create the Necessary VLANs

Configure VLAN10 and VLAN20 on SW0

SW0(config)#vlan 10
SW0(config-vlan)#name Sales
SW0(config)#vlan 20
SW0(config-vlan)#name Engineer

Step 3: Configure Trunks ports

Configure fa0/1 of SW0 as a trunk port

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SW0(config)#interface fa0/1
SW0(config-if)#switchport mode trunk
SW0(config-if)#switchport trunk allowed vlan 10,20
SW0(config-if)#no shutdown

Step 4: Configure access port

Put interface fa0/2 on VLAN10 and interface fa0/3 on VLAN20 and do those ports as access ports.

SW0(config)#interface fa0/3
SW0(config-if)#switchport mode access
SW0(config-if)#switchport access vlan 10
SW0(config-if)#no shutdown
SW0(config)#interface fa0/2
SW0(config-if)#switchport mode access
SW0(config-if)#switchport access vlan 20
SW0(config-if)#no shut

Step 5: Configure the host devices

Assign IP address to the two PCs as labelled in the network topology.

 Step 6: Check the routing table on the router

Check the routing on R0

R0#show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route
Gateway of last resort is not set is variably subnetted, 2 subnets, 2 masks
C is directly connected, GigabitEthernet0/0/0.10
L is directly connected, GigabitEthernet0/0/0.10 is variably subnetted, 2 subnets, 2 masks
C is directly connected, GigabitEthernet0/0/0.20
L is directly connected, GigabitEthernet0/0/0.20

As seen above, R0 has added some connected routes in its routing table.

Step 7: Verify Connectivity

To verify that the inter-vlan routing is working, send a ping from PC1 to PC0. As shown on the image below, the PCs is replying.

testing connectivity

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How to Configure Inter VLAN Routing on Cisco Layer 3 Switch

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