What is Legacy Inter-VLAN Routing? – Explained

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As we learned that each VLAN is usually working 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 VLAN inter-VLAN routing is required. The Legacy Inter-VLAN routing is the first solution for traffic between different VLAN. It relies on routers with multiple physical interfaces. All interface 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 and the router interface then accepts the traffic from the VLAN related with the switch interface it is connected to. Then the router sends the traffic to other VLANs connected to the other interfaces. The figure below illustrates the legacy inter-VLAN routing process.

Legacy Inter-VLAN routing

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

  • Host – 1 on VLAN 100 is communicating with Host – 2 on VLAN 200 through the Router.
  • The router has separate interfaces configured for both VLANs.
  • Both hosts are located in different VLANs, therefore both have different broadcast domains and cannot send traffic directly without a default gateway.
  • Host – 1 will check its ARP cache for MAC address of the default gateway, if the MAC address is found in the ARP cache of the host – 1, and then host – 1 will send the data packet to the router. But if the ARP cache has no MAC address of the default gateway, then the host -1 will generate the ARP request for the default gateway MAC address.
  • After getting the MAC address of the gateway; the 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 know to which interface it should forward the data towards 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 would flood 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 required on the router to configure Legacy inter-VLAN. The router can make a routing 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 to 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 as well as on the router. The figure below illustrates the Legacy inter-VLAN routing:

Legacy Inter-VLAN Routing – Switch Configuration

Legacy Inter-VLAN routing

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 assigned to VLAN-100 and the interfaces Fa0/6  to Interfaces Fa 0/10 have 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 just required to configured 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”.

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