If you’ve been in the networking field for a while, you’ve probably noticed how often the same questions come up around switch and router configurations—especially when it comes to how these devices connect and communicate within larger infrastructures. It’s not just theoretical; these setups form the backbone of real-world projects, particularly in corporate environments where multiple departments operate across different subnets and require seamless internet access.
Before we released our latest configuration guide, we kept hearing the same thing from engineers and IT managers: “How do we properly set up the link between the switch and the router? What’s the role of a Layer 3 switch when it’s acting as a gateway?” These aren’t just academic questions. They reflect day-to-day challenges in deploying scalable, secure, and high-performance networks.
This article breaks down one of the most common yet critical scenarios: interconnecting a Layer 3 switch with a router in a multi-department company. We’ll use Huawei hardware as a reference, but the principles apply across brands. Whether you’re looking to optimize your current setup or planning a new deployment, this guide offers actionable insights.
Understanding the Use Case: When Departments Need Cross-Network Access
Imagine a company where various departments are spread across different VLANs or IP segments. Each group needs internet access, but security and efficiency are paramount. The solution? Use a Layer 3 switch as the default gateway for all users and connect it to a router handling NAT and external routing.
This approach is widely adopted because it reduces latency, improves internal traffic management, and simplifies the scaling process. The switch handles inter-VLAN routing at high speed, while the router focuses on external communications.
Configuration Strategy: Keeping It Simple and Scalable
A clear configuration plan makes all the difference. Here’s how we approach it:
First, the Layer 3 switch is configured with VLAN interfaces (VLANIF) that act as gateways for each user segment. This allows devices in different VLANs to communicate without passing all traffic to the router.
Second, the switch serves as a DHCP server. Assigning IP addresses dynamically reduces manual configuration errors and streamlines network changes.
Third, the router is set up with NAT rules to translate private internal addresses into a public IP for internet access. Static or default routes ensure returning traffic finds its way back.
Switch Configuration Step-by-Step
Configuring the switch involves three main parts: user-facing interfaces, router-facing interfaces, and DHCP setup.
For user connections, access ports are assigned to VLANs. Each VLAN has a corresponding VLANIF interface with an IP address that serves as the default gateway for connected users.
The link to the router is typically a trunk port or access port belonging to a specific VLAN. The VLANIF for this VLAN is given an IP address on the same subnet as the router’s internal interface.
DHCP configuration involves creating address pools for each VLAN, specifying subnet masks, gateways, and DNS servers. This ensures users receive correct IP settings automatically.
Router Configuration: Tying the Network Together
On the router, start by configuring the internal interface—the one connected to the switch. Assign it an IP address within the same subnet as the switch’s router-facing VLANIF.
The external interface, connected to the internet, gets a public IP address. Then, set up a default route pointing to the ISP’s gateway.
Finally, configure NAT overload (PAT) to allow all internal users to share the public IP when accessing external resources. Don’t forget to add a return route to direct responses back to the correct internal subnet.
Validation: Does It Really Work?
After configuration, test connectivity. Set up PCs in different VLANs with static IPs or use DHCP. Then try pinging an external address—for example, a server with IP 200.0.0.1. If everything is set up correctly, both PC1 and PC2 should reach it without issues.
This proves that inter-VLAN routing and internet access are functional. For further verification, check the NAT table on the router to confirm translations are happening as expected.
But What Exactly Is VLANIF? How Is It Different from VLAN?
This is a recurring question we get at thunder-link.com, especially from folks who are more familiar with Layer 2 switching.
Think of VLAN as a virtual LAN—a broadcast domain that segments network traffic. It’s a Layer 2 concept, like putting labels on frames to keep groups separate. Devices in the same VLAN can communicate directly; those in different VLANs cannot.
VLANIF, on the other hand, is a virtual Layer 3 interface. You can assign it an IP address, and it acts as a gateway for that VLAN. It’s what allows traffic to move between VLANs without a physical router. In a way, it’s like having a built-in router port for each VLAN.
So why use VLANIF? Because it boosts performance. Instead of sending all inter-VLAN traffic to an external router, the switch handles it internally at wire speed. That means less latency, fewer bottlenecks, and better resource usage.
In the configuration above, we used multiple VLANIF interfaces—one per VLAN—to allow the switch to serve as the gateway for all users while also routing traffic between subnets.
This method is not only efficient but also highly scalable. As your network grows, you can add more VLANs and VLANIFs without major hardware changes.
Wrapping Up: Why the Right Configuration Matters
Getting the connection between switches and routers right isn’t just a technical exercise—it’s what keeps modern businesses connected, secure, and efficient. With the rise of hybrid work and cloud-dependent applications, a well-structured network is more important than ever.
The combination of Layer 3 switching and strategic router configuration lets you maintain control over internal traffic while providing robust internet access. It’s a balance of performance, security, and manageability.
If you’re looking to deepen your understanding or need specific hardware recommendations, visit us at thunder-link.com. We offer detailed guides, product comparisons, and configuration examples tailored to real-world scenarios. Because when it comes to networking, the right knowledge isn’t just power—it’s connectivity.
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