Configure and verify interVLAN routing (Router on a stick)

Exam: Cisco 200-301 - Cisco Certified Network Associate (CCNA)

By default the devices on the same VLAN only can communicate with each other. To allow communication between different VLANs you can configure inter-VLAN routing between them. In a traditional inter-VLAN routing setup, each router interface is connected to a separate network through a switch port, which is also associated with a specific VLAN. The router accepts traffic on its interface from one VLAN through switch and routes the traffic to another VLAN.

The following figure explains the scenario. The Switch SW1 has two VLANs VLAN 10 and VLAN 20 configured on interfacesFa/08 and Fa0/11 respectively. The switch ports Fa0/4 and Fa0/3, which are also the part of VLANs 10 and 20 respectively, are configured to router interfaces. Also, PC 1 is configured on VLAN 10 and PC 2 is configured on VLAN 20.

In this setup, you are configuring each VLAN on a separate router interface. You can configure multiple VLANs on a single interface using Router on stick inter-VLAN mechanism.

To configure Router on stick inter-VLAN mechanism,you need to configure the router’s interfaces as trunk links. Let us create a Router on stick setup where a single router interface is configured as a trunk link. The router will accept the tagged VLAN traffic on its trunk interface and route traffic between different VLANs.

To do this, you need to configure IP addresses on the router interfaces, which would be used by computers on VLAN 10 and VLAN 20 to accessthe devices connected to each other. The router may use routing table to forward data to the correct device, if the destination address is on adifferent VLAN.

Let us configure the setup shown in the diagram given below. The Router R1 is connected to switch SW1 on trunk port F0/3. VLANs 10 and 20 have also been added to switch SW1. PC1 and PC2 are connected to VLAN 10 and VLAN 20 respectively.

Configure VLANs 10 and 20 on Sw1

SW1#config t

SW1(config)#vlan 10

SW1(config-vlan)#vlan 20

SW1(config-vlan)#exit

Set interface fa0/3 to trunk mode. Remember, you cannot use the switchport mode dynamic auto or switchport mode dynamic desirable modes to create trunk on the router because the routers do not support dynamic trunking protocol.

SW1(config)#interface fa0/3

SW1(config-if)#switchport mode trunk

SW1(config-if)#end SW1#

Configure PC1 and PC2 with IP addresses and the default gateways

PC1:
IP Address: 192.168.10.2
Subnet: 255.255.255.0
Default Gateway: 192.168.1.1

PC2:
IP Address: 192.168.11.2
Subnet: 255.255.255.0
Default Gateway: 192.168.2.1

At this stage if you try to check the connectivity between PC1 and PC2, the connectivity will fail because the router that would connect the two VLANs is not configured,

Configure router with IP addresses to perform the inter-VLAN routingso that the router can route frames between VLAN 10 and VLAN 20.

Use interface interface_id.Subinterface_id command to create sub interfaces.

R1#config t

R1(config)#interface fa0/0.10

Next, assign VLAN ID using the encapsulation dot1q vlan_id subinterface.

R1(config-subif)#encapsulation dot1q 10

You also need to configure the router with the default gateways that you have configured on the PC1 and PC2 and assign IP address to the sub interfaces if the router.

R1(config-subif)#ip address 192.168.1.1 255.255.255.0

R1(config-subif)#interface fa0/0.20

R1(config-subif)#encapsulation dot1q 20

R1(config-subif)#ip address 192.168.2.1 255.255.255.0

R1(config-subif)#interface fa0/0

R1(config-if)#no shut

R1(config-if)#end

Verify Inter-VLAN Routing Configuration

View the router configuration using the show running-config command in privileged EXEC mode to verify the router configuration.

Router#show running-config

Interface Subinterface Gateway IP Address/Mask VLAN ID Router Encapsulation

Fa0/0 fa0/0.10 192.168.1.1/24 VLAN 10 encapsulation dot1q 10

Fa0/0 fa0/0.20 192.168.2.1/24 VLAN 20 encapsulation dot1q 20

Ping PC2 from PC1 to check if both the PCs in different VLANs can connect to each other.

C:\>ping 192.168.10.2

Pinging 192.168.10.2 with 32 bytes of data:

Reply from 192.168.10.2: bytes=32 time<10ms TTL=255

Ping statistics for 192.168.10.2:

Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),

Approximate round trip times in milli-seconds:

Minimum = 0ms, Maximum = 0ms, Average = 0ms

C:\>ping 192.168.10.2

Pinging 192.168.10.2 with 32 bytes of data:

Request timed out.

Reply from 192.168.10.2: bytes=32 time<10ms TTL=255

Ping statistics for 192.168.10.2:

Packets: Sent = 4, Received = 3, Lost = 1 (25% loss),

Approximate round trip times in milli-seconds:

Minimum = 0ms, Maximum = 0ms, Average = 0ms

Verifying the VLAN Configuration

Router>show vlan

Virtual LAN ID: 10 (Inter Switch Link Encapsulation)

vLAN Trunk Interface: FastEthernet0/0.10

Protocols Configured: Address: Received: Transmitted:

IP 192.168.1.1 0 0

Virtual LAN ID: 20 (Inter Switch Link Encapsulation)

vLAN Trunk Interface: FastEthernet0/0.20

Protocols Configured: Address: Received: Transmitted:

IP 192.168.2.1 0 0

Troubleshooting Inter-VLAN Routing Configuration

If PC1 and PC2 are not able to connect each other and if you are not able to share resources of VLAN 20 from VLAN 10 and Vice versa. Then the possible causes may be:

The routerinterface on switch is not configured as a trunk and the port is left in the default VLAN mode. To resolve the problem, use the switchport mode trunk interface command on switch to enable the connectivity between the switch and router.

Make sure PCs are in correct VLANs and are configured with correct IP addresses and default gateway addresses
Ensure that the router is configured with the correct sub-interfaces that match the default gateway of the PCs

Example Question

Q1. In which of the following ways does Router on stick inter-VLAN method configured?

You can configure multiple VLANs on a single interface using Router on stick inter-VLAN mechanism.
In the router on stick inter-VLAN method, each router interface is connected to a separate network through a switch port, which is also associated with a specific VLAN.
In the router on stick inter-VLAN method, a server is configured to perform the routing between the two VLANs
None of the above.

Answer A

When Router on stick inter-VLAN method is configured, you can configure multiple VLANs on a single interface.

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