CCNA RnS Article #25: Inter-VLAN Routing

CCNA RnS Article #25: Inter-VLAN Routing

In this article, I am going to talk about Inter-VLAN routing. Before I talk about that let’s understand why we need that through a simple real-life example. We build networks to save cost by allowing network sharing. 

Let’s say in a large network we have multiple switches and VLANs implemented – there are few resources in the network i.e printers, File-servers, etc. need to be accessible to every user irrespective of it’s VLAN or location in the network.

Inter-VLAN Routing
Fig 1.1- Inter-VLAN routing

Till now we discussed layer 2 frame forwarding of the switch where users in the same VLAN and subnets can talk to each other without any problem. If we come across a requirement where one VLAN (Shared Resource) needs to communicate to multiple VLANs such as Corporate, Guest, and so on, we need a device that can allow inter-VLAN communication.

In a layer 2 environment, this VLAN setup conceptually looks like Figure 1.2 and doesn’t allow to talk to each other.

Figure 1.2 : Layer 2 Switch - No Inter-VLAN communication

Each VLAN represents a separate broadcast domain and uses a unique IP subnet to allow users in the same subnet to talk to each other. As per Figure 1, both file servers and printers can talk to each other as they are part of the same VLAN, and Corporate users can talk to other corporate users only. To provide communication among all the users and shared devices, we need a layer 3 device called a router. This is how the same network (Figure 1) would look to make communication happen -

Figure 1.3: Inter-VLAN Routing w/ Router

According to Figure 1.3, we have a router in between to make inter-VLAN communication possible. Router has 2 interfaces, and each interface is connected to different VLANs that need cross-communication. Router Ethernet 0/0 interface is the default gateway for all shared resources and is in the same VLAN. Router Ethernet 0/1 interface is the default gateway for corporate devices.

A corporate user wants to access the file server, the device NIC will see that the destination address is on a different network (Subnet 1) so it needs to be handed over to the default gateway. Router receives the frame on Ethernet 0/1 and checks the interface where the destination network (Subnet 1) exists, using its routing table it comes to know that subnet 1 can be reached through Ethernet 0/0. Traffic received by switch 1 and based on the destination MAC address information handed over to the file server.

Figure 1.4: Inter-VLAN routing using Router

It works fine however it is not a scalable solution. As you may need multiple physical interfaces to perform inter-VLAN routing – trunking between router and switch can also work but it shares the physical interface bandwidth.

The use of a router in the previous scenario was to just explain the inter-VLAN routing concept in brief. However, in a real-life scenario, we perform inter-VLAN routing using a special switch. This special switch can perform both routing and switch. Therefore, it is called a multi-layer switch. Using a multi-layer switch is a simple and cost-effective solution to implement inter-VLAN routing in a campus network. Hope you find this informative – I’ll cover switching way of inter-VLAN routing soon in an upcoming article.

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