InterVLAN Routing: The Ultimate Guide
VLANs divide large networks into smaller parts, which helps reduce network congestion and facilitate troubleshooting. By default, VLANs don’t allow access to resources from other VLANs. However, if you want to access resources in other VLANs, then the concept of Intervlan routing is used to enable this. InterVLAN routing enables communication between different VLANs within a network. It allows access to resources in other VLANs when needed. Intervlan routing is possible with a Layer 3 device, which should be either a router or a layer 3 switch.
Methods for InterVlan Routing
There are three common methods to implement intervlan routing:
1. Router on a stick
In this method, the router has only one interface, but is configured with multiple sub-interfaces for each VLAN.
How it Works
- A trunk link is used between the router and the switch
- Each sub-interface is associated with the respective vlan. Also, each sub-interface has its IP address.
- When a device in one vlan sends traffic to the other vlan, then the switch forwards that traffic to the router. The router then sends it back to the switch with its sub-interfaces to the respective vlan.
Lab Topology for Router on a Stick
The following scenario for router on a stick will be used. With the help of which we will configure the router on a stick configuration.
Create three VLANs and then assign them to the appropriate interfaces,
Create a trunk link between router and the switch;
First we check the connectivity among VLANs with the ping command from either of one VLANs e.g. from VLAN 20 to VLAN 10 and VLAN 30.
The ping was not successful, because we didn’t have a router for inter-vlan routing. Now configure the router for inter-vlan routing. Create three sub-interfaces on the router and then assign them IP as well as VLANs to them.
Ping other VLANs, but before pinging, set the default gateways of all the PCs as below;
Ping VLAN 10 and VLAN 30 from VLAN 11’s PC;
2. Intervlan Routing using Layer 3 Switch
A layer 3 switch is used instead of a router for inter-vlan communication. It eliminates the requirement of an external router.
How It Works?
. IP addresses are assigned to each vlan through Switch Virtual Interface (SVI)
. The switch processes inter-vlan routing and forwards packets between the VLANs without sending it to the external router.
. We can use dynamic routing protocols such as RIP, EIGRP, or OSPF, for scalability.
SVI Configuration
We will use the following lab topology for SVI (switched virtual interface), which consists of one layer 3 switch with two VLANs.
Configure te switch for intervlan routing; Create two VLANs e.g. VLAN 11 and 12. Assign interfaces from fa 0/1 and fa 0/2 to VLAN 11, and fa 0/11, fa 0/12 to VLAN 12.
Enable routing on the switch and at the last assign IP addresses to the both VLANs;
After configuring “ip routing” on the switch, ping from VLAN 11 to VLAN 12.
3. Traditional Approach
We associate each VLAN with the dedicated physical interface of the router. The traffic is moving among different VLANs through these interfaces. However, if there are more VLANs, then we will need that particular physical interfaces as the number of VLANs.
