What is Legacy Inter-VLAN Routing? – Explained

As we learned, each VLAN usually works on its subnet. The network switches mainly work at layer 2 of the OSI model, so they do not examine the logical addresses. Therefore, for traffic between VLANs, inter-VLAN routing is required. The Legacy Inter-VLAN routing is the first solution for traffic between different VLANs. It relies on routers with multiple physical interfaces. All interfaces had to be connected to a separate network and configured with a separate subnet.

The legacy inter-VLAN routing connects different physical router interfaces to different physical ports on the switch. The switch ports connected to the router must be placed in access mode.

Each physical interface of the router is assigned to a different VLAN. The router interface then accepts traffic from the VLAN related to the switch interface it is connected to. Then, the router sends the traffic to other VLANs connected to the different interfaces. The figure below illustrates the legacy inter-VLAN routing process.

We can see that host 1 is VLAN 100, and host 2 is VLAN 200. So if host 1 wants to send data to host 2, the following steps would be.

• Host – 1 on VLAN 100 communicates with Host – 2 on VLAN 200 through the Router.
• The router has separate interfaces configured for both VLANs.
• Both hosts are in different VLANs, so they have different broadcast domains and cannot send traffic directly without a default gateway.
• Host 1 will check its ARP cache for the default gateway’s MAC address. If the MAC address is found in the cache, host 1 will send the data packet to the router. However, if the ARP cache does not have the default gateway’s MAC address, host 1 will generate the ARP request for it.
• After getting the gateway’s MAC address, host -1 will send the packet to its default gateway (the router interface fa0/0). When the router receives the frame, it compares the destination IP address by referring to its routing table to determine which interface it should forward the data to the destination host.
• The router then forwards an ARP request out the interface connected to the destination VLAN; when the switch receives the message, it floods it to its ports, and in this case, host – 2 would reply with its MAC address.
• Router – 1 would then use this information to send it to host – 2 as a unicast frame.
• December 28, 2018

Configure Legacy Inter-VLAN Routing

Multiple Physical interfaces are required on the router to configure Legacy inter-VLAN. The router can route with each of its physical interfaces connected to a unique VLAN. Configure each physical interface with the unique IP address for the subnets related to the particular VLAN. 

After configuring the IP address on physical interfaces, each device connected to the LAN can communicate with the router using this physical interface. The router would be the gateway for each device on the VLAN. All the VLAN can communicate with each other without configuring any routing protocol on the router.  The legacy Inter-VLAN routing required configuration on the switch and the router. The figure below illustrates the Legacy inter-VLAN routing:

Legacy Inter-VLAN Routing – Switch Configuration

To configure legacy inter-VLAN routing starts by configuring the switch. As shown in the figure, the Router is connected to switch ports Fa0/2 and Fa0/7, which have been configured for VLANs 100 and 200, accordingly.

Use the following command to create VLAN and assign ports to that VLAN. Remember that issue the command in global configuration mode. The port must be in access mode

In this example, the interfaces Fa0/1 to Fa05 have been assigned to VLAN-100 and the interfaces Fa0/6  to Interfaces Fa 0/10 have been assigned to VLAN 200. Using the name command have assigned the name to VLANs and finally using the wr (write) command save the work into the startup configuration file.

We have used the command “do wr” because we are in global configuration mode. The command is originally not using in global configuration mode. So, if we are in “User Privileged Mode” then we would use the command “wr or write”. We can also use the copy “running-config startup-config” instead of the “wr” command. Watch the following video for Legacy Inter-VLAN routing configuration:

Legacy Inter-VLAN Routing – Router Configuration

In the Legacy Inter-VLAN routing, there are no static or dynamic protocols needed for the routing. We are just required to configure the IP addresses of the router according to the subnet of the connected VLANs. We can configure the IP address of the interface using “IP address <ip address subnet mask> command in global configuration mode. Remember that the switch must be in the status of “no shutdown”.

Conclusion:

Legacy Inter-VLAN routing is a foundational solution for facilitating communication between different VLANs within a network. As network switches operate primarily at Layer 2 of the OSI model and do not inherently support inter-VLAN communication, routers with multiple physical interfaces become essential for routing traffic between VLANs. By configuring each router interface with a unique VLAN and subnet, Legacy Inter-VLAN routing enables devices within those VLANs to communicate through the router, serving as their gateway. Although this method requires separate physical interfaces and subnets, it offers a straightforward approach to inter-VLAN routing.

FAQs:

Q. What is Legacy Inter-VLAN Routing?

A. Legacy Inter-VLAN Routing is a method of facilitating communication between different VLANs in a network using routers with multiple physical interfaces. It involves configuring each router interface to correspond with a unique VLAN and subnet, allowing devices within those VLANs to communicate through the router.

Q. Why is Legacy Inter-VLAN Routing Necessary?

A. Network switches operate at Layer 2 and do not examine logical addresses, making inter-VLAN communication challenging. Legacy Inter-VLAN Routing addresses this limitation by utilizing routers with multiple physical interfaces to route traffic between VLANs.

Q. How does Legacy Inter-VLAN Routing Work?

A. Each physical router interface is assigned to a different VLAN, and devices within those VLANs communicate through the router. When a device needs to communicate with a device in another VLAN, the router serves as the gateway. The router compares destination IP addresses, forwards traffic between VLANs, and uses ARP requests to determine MAC addresses.

Q. What is the Configuration Process for Legacy Inter-VLAN Routing?

A. Multiple physical interfaces on the router are configured with unique IP addresses for the subnets related to each VLAN. Switch ports are configured in access mode and assigned to specific VLANs. The router interfaces connect to corresponding switch ports, establishing the inter-VLAN communication path.

Q. Do I Need Routing Protocols for Legacy Inter-VLAN Routing?

A. No, Legacy Inter-VLAN Routing does not require static or dynamic routing protocols. The router is configured with IP addresses for each VLAN, and devices communicate through the router without additional routing protocols.

Q. Can Legacy Inter-VLAN Routing be Configured on Switches?

A. Legacy Inter-VLAN Routing primarily involves router configuration. Switches are configured to assign ports to specific VLANs, and the router handles the routing with multiple physical interfaces.