by Darin Knobbe on Nov 18, 2021 9:09:59 AM
Download the Guide Table of Contents In a Layer 2 switched environment, VLANs separate devices into different collision domains and Layer 3 subnets. Devices in the same VLAN can communicate without any routing, and devices in different VLANs require routing to communicate with each other. L2 switches require an L3 routing device to communicate between different VLANs, and the device is either an external router or Layer 3 module on the same chassis. Most of the Cisco switches have routing capabilities within the switch. When the switch receives a packet, it determines that it belongs to another VLAN, and the switch sends the packet to the appropriate port on the other VLAN. This blog will discuss inter-VLAN routing configuration examples in Cisco routers and switches.
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What Is Inter-VLAN Routing?
VLANs are used to segment Layer 2 networks. Hosts from one VLAN cannot communicate with hosts in another VLAN unless a router or a Layer 3 switch provides routing services.
Inter-VLAN routingis the technology for communicating network traffic from one VLAN to another VLAN.
There are two well-known inter-VLAN routing options:
- Router-on-a-Stick: This is a suitable solution for small- to medium-sized networks.
- Switched Virtual Interfaces (SVIs): This is the most scalable solution for medium to large organizations, and this solution needs a Layer 3 switch.
Router-on-a-Stick Inter-VLAN Routing Method With a Cisco Router
The "router-on-a-stick" inter-VLAN routing technique needs only one physical interface to route traffic among multiple VLANs on a network. A router's physical interface is configured as an 802.1Q trunk and connected to the Layer 2 switch's trunk interface.
The router interface is configured using subinterfaces for passing multiple VLANs in one interface. The configured subinterfaces are software-based virtual interfaces, and each subinterface is associated with a single physical Ethernet interface. Each sub-interface is configured for different subnets that correspond to their VLAN assignment, which facilitates logical routing.
When VLAN-tagged traffic enters the router interface, the traffic is forwarded to the VLAN subinterface according to the VLAN tag ID. After a routing decision is performed based on the destination address, the router determines the egress interface for the traffic.
NOTE: The router-on-a-stick inter-VLAN method does not scale beyond 50 VLANs.
Router-on-a-Stick Inter-VLAN Routing Configuration Example
This section describes how to configure the router-on-a-stick inter-VLAN routing method. You can see in the image below that the router is connected with a switch using a single interface, and can pass multiple VLANs using that interface via a router-on-a-stick inter-VLAN routing configuration.
The router GigabitEthernet 0/0/0 interface is connected to the Layer 2 switch Ethernet 1/1 port. The Ethernet 1/1 is a trunk link that is required to forward traffic within and between VLANs.
To route between VLANs, the Router GigabitEthernet 0/0/0 interface is logically divided into two subinterfaces, as shown in the table below. The table also shows the two VLANs that will be configured on the switch.
| Subinterface | VLAN | IP Address |
G0/0/0.100 | 100 | 10.1.100.1/24 |
G0/0/0.200 | 200 | 10.1.200.1/24 |
Assume that the Router and Switch have basic configurations. Currently, PC1 and PC2 cannot ping each other because they are on different networks.
To enable the PCs to communicate, we need to configure VLANs and trunking in the switch level, and the router must be configured for inter-VLAN routing.
VLAN and Trunking Configuration in Switch
Complete the following steps to configure Switch with VLANs and trunking:
VLAN Configuration
- Step 1 -VLANs Creation:First, the VLANs are created and named, as shown in the below configuration.
- Step 2 -Configure Access Ports:Next, port Eth1/2 connecting to PC1 is configured as an access port in VLAN 100, as shown below. Assume PC1 has been configured with the correct IP address, netmask, and default gateway1.100.1.
Access Port Configuration
- Step 3 -Trunking Port Configuration: Configure the port Eth1/1 as a trunk port like the below configuration.
Trunking Port Configuration
Router Subinterface Configuration
For each VLAN, you need to create a subinterface in the router-on-a-stick method. A subinterface can be created using the interface_id.subinterface_id format in the global configuration mode. The subinterface syntax is the physical interface followed by a period and a subinterface number. Although not required, it is common to match the subinterface number with the VLAN number.
After creating the subinterface, enable the interface with the "no shutdown" command in the interface configuration mode. All the subinterfaces are disabled if the physical interface is disabled.
In the below configuration, the Router G0/0/0 subinterfaces are configured for VLANs 100 and 200.
Verify Connectivity Between PC1 and PC2
After the switch trunk and the router subinterface configurations, the router-on-a-stick configuration is complete. The configuration can be verified from the PC, router, and switch.
From PC1, verify connectivity to a host in another VLAN using theping command.
Windows PC IP Address
The output confirms the IPv4 address and default gateway of PC1. Next, usepingto verify connectivity with PC2, as shown in the below output. Theping reply confirms that inter-VLAN routing is working.
Router-on-a-Stick Inter-VLAN Routing Verification
Use the below show commands to verify and troubleshoot the router-on-a-stick configuration.
- show ip route
- show ip interface brief
- show interfaces
As shown in the below output, verify that the subinterfaces appear in the router's routing table by using the "show ip route" command. Notice that there are two connected routes (C) and their respective exit interfaces for each routable VLAN. The output confirms that the subnets, VLANs, and subinterfaces are active.
Another useful router command isto show the ip interface brief, as shown below. The output confirms that the subinterfaces have the correct IPv4 address configured and that they are operational.
Subinterfaces can be verified using theshow interfacessubinterface-command, as shown in the below output.
In Summary: Inter-VLAN Routing FAQs
What is inter-VLAN routing?
Inter-VLAN routing enables routers or Layer 3 switches to route traffic between VLANs.
Why is inter-VLAN routing necessary?
While it’s sometimes necessary to isolate VLANs and disallow any traffic between them, often some traffic needs to flow between VLANs so certain services and systems are available on any network segment. That’s where inter-VLAN routing comes in.
What are the benefits of inter-VLAN routing?
Inter-VLAN routing allows for communication between different VLANs, which is necessary for certain services and systems to be available across network segments. It also helps in managing network traffic efficiently.
What devices are used in inter-VLAN routing?
Routers or Layer 3 switches are typically used to implement Inter-VLAN routing. These devices can route traffic between VLANs while preserving broadcast domains.
What is the ‘Router-on-a-Stick’ model in inter-VLAN routing?
The ‘Router-on-a-Stick’ model is a common design pattern for Inter-VLAN routing. In this model, a single interface on a router is used to route traffic between multiple VLANs.
How Does Inter-VLAN Routing Improve Network Security?
By segregating the network into different VLANs and controlling the traffic between them using Inter-VLAN routing, network administrators can add an additional layer of security. This can help prevent unauthorized access and contain potential threats within a single VLAN.
