Packet Tracer - Configure Router-on-a-stick Inter-vlan Routing

Configuring Router-on-a-Stick Inter-VLAN Routing with Packet Tracer

In modern network design, segmenting a network into Virtual Local Area Networks (VLANs) is crucial for enhancing security, improving network performance, and simplifying network management. However, VLANs, by default, cannot communicate with each other. This is where inter-VLAN routing comes into play. One popular and cost-effective method for enabling inter-VLAN routing is using a "Router-on-a-Stick" configuration. This article delves into configuring Router-on-a-Stick inter-VLAN routing using Cisco Packet Tracer, a powerful network simulation tool.

Understanding VLANs and Inter-VLAN Routing

Before diving into the configuration steps, it's important to understand the basics of VLANs and inter-VLAN routing.

• VLANs (Virtual Local Area Networks): VLANs are logically segmented broadcast domains within a physical network. They allow you to group devices together, regardless of their physical location, based on factors like department, function, or security requirements.
• Inter-VLAN Routing: This is the process of enabling communication between different VLANs. Without inter-VLAN routing, devices in separate VLANs cannot directly communicate, even if they are connected to the same physical switch.
• Router-on-a-Stick: This is a type of inter-VLAN routing configuration where a single physical router interface is used to route traffic between multiple VLANs. This is achieved by creating subinterfaces on the router interface, each representing a different VLAN.

Benefits of Using VLANs

Implementing VLANs offers numerous benefits:

• Enhanced Security: VLANs isolate traffic, preventing broadcast traffic from one VLAN from affecting other VLANs. This limits the scope of security breaches and enhances overall network security.
• Improved Performance: By segmenting the network, VLANs reduce broadcast traffic and network congestion, leading to improved network performance.
• Simplified Management: VLANs make it easier to manage network resources and apply policies to specific groups of users or devices.
• Flexibility and Scalability: VLANs provide the flexibility to easily add, move, or change network devices without requiring physical rewiring.

Limitations of Router-on-a-Stick

While Router-on-a-Stick is a cost-effective solution, it does have some limitations:

• Single Point of Failure: Because all inter-VLAN traffic passes through a single router interface, this interface becomes a potential single point of failure.
• Performance Bottleneck: The single physical interface can become a bottleneck if there is a high volume of inter-VLAN traffic.

Setting Up the Network Topology in Packet Tracer

Let's begin by setting up the network topology in Cisco Packet Tracer. This topology will consist of:

• One Router (e.g., 2911)
• One Switch (e.g., 2960)
• Two PCs (representing different VLANs)

Here's a step-by-step guide to creating the topology:

1. Open Cisco Packet Tracer.
2. Select Devices: From the device toolbar at the bottom left, select the following devices:
   • Router: Click on the "Routers" icon and choose a router (e.g., 2911). Drag and drop it onto the workspace.
   • Switch: Click on the "Switches" icon and choose a switch (e.g., 2960). Drag and drop it onto the workspace.
   • PCs: Click on the "End Devices" icon and choose two PCs. Drag and drop them onto the workspace.
3. Connect Devices: Use the "Connections" tool (the lightning bolt icon) to connect the devices:
   • Connect PC0 to Switch port FastEthernet0/1.
   • Connect PC1 to Switch port FastEthernet0/2.
   • Connect the Router GigabitEthernet0/0 to the Switch port FastEthernet0/24.
4. Label Devices: You can label the devices to make the topology easier to understand. Click on a device and change its display name in the configuration window. For example, you can name the router "Router1," the switch "Switch1," and the PCs "PC0" and "PC1."

Configuring the Switch for VLANs

Next, we need to configure the switch to create and assign VLANs to the appropriate ports. In this example, we will create two VLANs: VLAN 10 and VLAN 20.

1. Access the Switch CLI: Click on the switch to access its configuration window. Go to the "CLI" tab.
2. Enter Enable Mode: Type enable and press Enter.
3. Enter Configuration Mode: Type configure terminal and press Enter.
4. Create VLANs:
   • Create VLAN 10: vlan 10 and press Enter.
   • Name VLAN 10: name VLAN10 and press Enter.
   • Create VLAN 20: vlan 20 and press Enter.
   • Name VLAN 20: name VLAN20 and press Enter.
5. Assign Ports to VLANs:
   • Assign port FastEthernet0/1 to VLAN 10:

     interface FastEthernet0/1
     switchport mode access
     switchport access vlan 10
     

     Press Enter after each line.
   • Assign port FastEthernet0/2 to VLAN 20:

     interface FastEthernet0/2
     switchport mode access
     switchport access vlan 20
     

     Press Enter after each line.
6. Configure the Trunk Port: The port connecting the switch to the router (FastEthernet0/24) needs to be configured as a trunk port to allow traffic from multiple VLANs to pass through it.

   interface FastEthernet0/24
   switchport mode trunk
   switchport trunk encapsulation dot1q
   switchport trunk allowed vlan 10,20
   

   Press Enter after each line. The switchport trunk encapsulation dot1q command specifies the trunking protocol (802.1Q). The switchport trunk allowed vlan 10,20 command allows VLANs 10 and 20 to pass over the trunk.
7. Exit Configuration Mode: Type end and press Enter.
8. Verify VLAN Configuration: Type show vlan brief and press Enter. This command will display a summary of the VLAN configuration, showing which ports are assigned to which VLANs.

Configuring the Router for Inter-VLAN Routing

Now, let's configure the router to enable inter-VLAN routing using the Router-on-a-Stick method. This involves creating subinterfaces for each VLAN and assigning IP addresses to them.

1. Access the Router CLI: Click on the router to access its configuration window. Go to the "CLI" tab.
2. Enter Enable Mode: Type enable and press Enter.
3. Enter Configuration Mode: Type configure terminal and press Enter.
4. Configure Subinterfaces: We will create two subinterfaces on the GigabitEthernet0/0 interface: GigabitEthernet0/0.10 for VLAN 10 and GigabitEthernet0/0.20 for VLAN 20.
   • Configure subinterface GigabitEthernet0/0.10:

     interface GigabitEthernet0/0.10
     encapsulation dot1Q 10
     ip address 192.168.10.1 255.255.255.0
     

     Press Enter after each line. The encapsulation dot1Q 10 command specifies that this subinterface will handle traffic for VLAN 10. The ip address 192.168.10.1 255.255.255.0 command assigns the IP address 192.168.10.1 to the subinterface with a subnet mask of 255.255.255.0.
   • Configure subinterface GigabitEthernet0/0.20:

     interface GigabitEthernet0/0.20
     encapsulation dot1Q 20
     ip address 192.168.20.1 255.255.255.0
     

     Press Enter after each line. The encapsulation dot1Q 20 command specifies that this subinterface will handle traffic for VLAN 20. The ip address 192.168.20.1 255.255.255.0 command assigns the IP address 192.168.20.1 to the subinterface with a subnet mask of 255.255.255.0.
5. Enable the Physical Interface: Enable the physical interface GigabitEthernet0/0.

   interface GigabitEthernet0/0
   no shutdown
   

   Press Enter after each line. The no shutdown command activates the interface.
6. (Optional) Configure DHCP Server: You can configure the router to act as a DHCP server for each VLAN. This will automatically assign IP addresses to the PCs in each VLAN.
   • Configure DHCP for VLAN 10:

     ip dhcp pool VLAN10
     network 192.168.10.0 255.255.255.0
     default-router 192.168.10.1
     dns-server 8.8.8.8
     

     Press Enter after each line. The ip dhcp pool VLAN10 command creates a DHCP pool named VLAN10. The network 192.168.10.0 255.255.255.0 command specifies the network address and subnet mask for VLAN 10. The default-router 192.168.10.1 command sets the default gateway for VLAN 10 to the IP address of the subinterface. The dns-server 8.8.8.8 command specifies the DNS server address.
   • Configure DHCP for VLAN 20:

     ip dhcp pool VLAN20
     network 192.168.20.0 255.255.255.0
     default-router 192.168.20.1
     dns-server 8.8.8.8
     

     Press Enter after each line. The ip dhcp pool VLAN20 command creates a DHCP pool named VLAN20. The network 192.168.20.0 255.255.255.0 command specifies the network address and subnet mask for VLAN 20. The default-router 192.168.20.1 command sets the default gateway for VLAN 20 to the IP address of the subinterface. The dns-server 8.8.8.8 command specifies the DNS server address.
7. Exit Configuration Mode: Type end and press Enter.
8. Verify Router Configuration: Type show ip interface brief and press Enter. This command will display a summary of the interface configuration, showing the IP addresses assigned to the subinterfaces.

Configuring the PCs

Now that the switch and router are configured, let's configure the PCs to obtain IP addresses from the DHCP server (if configured) or manually assign them IP addresses.

1. Configure PC0:
   • Click on PC0 to access its configuration window.
   • Go to the "Desktop" tab.
   • Click on "IP Configuration."
   • Select "DHCP" if you configured DHCP on the router. Otherwise, select "Static" and enter the following information:
     • IP Address: 192.168.10.10 (or any other available IP address in the 192.168.10.0/24 range)
     • Subnet Mask: 255.255.255.0
     • Default Gateway: 192.168.10.1
   • Close the IP Configuration window.
2. Configure PC1:
   • Click on PC1 to access its configuration window.
   • Go to the "Desktop" tab.
   • Click on "IP Configuration."
   • Select "DHCP" if you configured DHCP on the router. Otherwise, select "Static" and enter the following information:
     • IP Address: 192.168.20.10 (or any other available IP address in the 192.168.20.0/24 range)
     • Subnet Mask: 255.255.255.0
     • Default Gateway: 192.168.20.1
   • Close the IP Configuration window.

Testing Connectivity

Finally, let's test the connectivity between the PCs to ensure that inter-VLAN routing is working correctly.

1. Open Command Prompt on PC0:
   • Click on PC0 to access its configuration window.
   • Go to the "Desktop" tab.
   • Click on "Command Prompt."
2. Ping PC1: Type ping 192.168.20.10 and press Enter.
3. Verify Connectivity: If the ping is successful, you should see replies from PC1's IP address. This indicates that inter-VLAN routing is working correctly.
4. Repeat the process on PC1, pinging PC0 (ping 192.168.10.10) to confirm bidirectional communication.

If the ping is unsuccessful, double-check the following:

• VLAN configuration on the switch.
• Subinterface configuration on the router.
• IP address configuration on the PCs.
• Trunk port configuration on the switch.

Troubleshooting Common Issues

Here are some common issues you might encounter and how to troubleshoot them:

• PCs cannot obtain IP addresses:
  • Ensure that DHCP is configured correctly on the router.
  • Verify that the VLAN configuration on the switch is correct.
  • Check if the PCs are configured to obtain IP addresses automatically.
• PCs can obtain IP addresses, but cannot ping each other:
  • Verify that the subinterface configuration on the router is correct.
  • Ensure that the trunk port is configured correctly on the switch.
  • Check if there are any access control lists (ACLs) blocking traffic between the VLANs.
• Trunk port not functioning:
  • Ensure that the encapsulation type is configured correctly (dot1q).
  • Verify that the allowed VLANs are configured correctly on the trunk port.
  • Check for port errors on the switch and router interfaces.

Advanced Configuration Options

Once you have the basic Router-on-a-Stick configuration working, you can explore some advanced configuration options:

• Access Control Lists (ACLs): Use ACLs to control traffic flow between VLANs. For example, you can create an ACL to allow traffic from VLAN 10 to VLAN 20, but deny traffic from VLAN 20 to VLAN 10.
• Quality of Service (QoS): Implement QoS to prioritize certain types of traffic over others. For example, you can prioritize voice traffic over data traffic to ensure good call quality.
• Routing Protocols: In more complex networks, you can use routing protocols like OSPF or EIGRP to dynamically learn routes and distribute routing information.

Conclusion

Configuring Router-on-a-Stick inter-VLAN routing with Packet Tracer is a valuable skill for network administrators and engineers. This configuration allows you to segment your network into VLANs, improving security, performance, and manageability, while utilizing a single router interface for inter-VLAN communication. By following the steps outlined in this article, you can successfully implement Router-on-a-Stick inter-VLAN routing in your Packet Tracer simulations and gain a better understanding of network segmentation and routing principles. Remember to carefully plan your network topology, configure the switch and router correctly, and test the connectivity thoroughly to ensure that everything is working as expected. With a solid understanding of VLANs and inter-VLAN routing, you can design and manage more efficient and secure networks.