What Is InterVLAN Routing?

by Girard Kavelines
What Is InterVLAN Routing

In life, communication is essential in making sure things are relayed properly to other parties – that critical information can be given and delivered in a timely manner and so much more. We communicate with our partners everyday what needs to be done around the house, what’s needed from the grocery store, when the kid’s games are and other important information to keep our day to day lives running smoothly. On a network we rely on the same factors. Using different protocols such as BGP, OSPF, and EIRGP to name a few communication is needed in order to allow packets from one network to another to be forwarded. While those protocols are vital to an infrastructure, we need the same type of system for our VLANs (virtual LANs) to assure traffic is being forwarded properly and that’s where interVLAN routing comes in.

I do enjoy interVLAN routing so I’m going to explain what it is and some of the many types of interVLAN routing that would best suit your infrastructure’s needs so let’s take a look:

InterVLAN Routing – The Concept

For me, especially while studying for my CCNA, I’ve always loved not just the different types of protocols, but how traffic flows as well. With interVLAN routing, it allows a device or a configuration to communicate with another VLANs and forward traffic. Normally VLANs are implemented to separate Layer 2 networks for a ton of different reasons – hosts cannot communicate with other hosts in separate VLANs unless a Layer 3 switch or a router is there to provide those services. So, for example, if you had two users across two different departments in your organization, on their respective VLANs – they wouldn’t be able to communicate with each other. But if someone from one department needed to send information to someone from another department – how would they achieve this? InterVLAN routing is the answer. Now that we know what it is, let’s look at some different ways to implement interVLAN routing.

Legacy InterVLAN Routing

Just as its name suggests, legacy interVLAN routing is definitely an older yet original method used in interVLAN routing. While not the most efficient for today’s networks, there is some upside to it. For one, there’s ease of configuration in setting it up, but while that’s one upside there are some other methods required. Setting up router interfaces with each one connecting to a switchport with different VLANs, this route will require additional cabling later on when expanding the network.

Router On A Stick

Router on a stick, also known as a one-armed router, is a router that has a single physical or logical connection to a network. This method is used primarily with most small to medium sized networks. Router on a stick method allows multiple VLANs to go over a single connection without any Layer 3 switching needed. Some setbacks to this are its known for being a more complex solution to setup by having the traffic between VLANs setup go through the router and out of the same port. It can cause congestion, slow the network and cause performance issues that you don’t want.

Switched Virtual Interface (SVI)

Switched virtual interfadce is one of the most popular methods used especially when scaling larger enterprise networks. These are created the same way most management VLANs are configured. This acts in the same function as a router interface would but done virtually. With a ton of upside to this method, it is why most enterprises focus on this route. Everything is connected through hardware and patched in – with all of the routing and switching being done – no external links are needed. Latency is also lower cause no data needs to leave the switch.

With interVLAN routing, there’s many different approaches to how you can implement within your organization. Allowing traffic to communicate across other areas in an organization is key to any companies thriving success. But what if that traffic gets congested and crashes? Or we run into an issue with forwarding that traffic to another VLAN? In my next blog, I’ll explain some keys areas to focus on when troubleshooting VLAN issues and more.

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