L3 Routing Topology: SVI and IP Essentials

The L2/L3 Boundary: A Core Design Decision

The most fundamental decision in a campus network design is determining where Layer 2 (switching) ends and Layer 3 (routing) begins. This decision impacts scalability, convergence time, and complexity.

To understand the options, let's explore a high-level design article.

As you read, you'll see two primary models:

1. Routed Access: Each access-layer switch is a Layer 3 device. The uplink to the distribution layer is a routed (L3) port. The default gateway for hosts connected to that switch resides on the switch itself. This model offers very fast convergence and simplifies the topology by eliminating L2 loops and spanning-tree, but requires L3-capable switches everywhere.
2. L2 Access / L3 Distribution (Multi-Tier): The access layer is purely Layer 2. Access switches connect to the distribution layer via L2 trunk ports. All routing intelligence is centralized on one or more L3 distribution switches. This is a very common, flexible, and cost-effective design for many enterprise environments.

For our Aperture Logistics design, we will adopt the L2 Access / L3 Distribution model. This aligns well with the scale of the organization (~1000 users) and centralizes the complex routing functions, making management more straightforward. In this model, one or more powerful L3 switches at the core/distribution layer will be the routing workhorse for the entire campus.

The Layer 3 Switch: Your Network's Routing Hub

Since we've chosen to centralize routing, the Layer 3 switch is the star of the show. But what exactly is it? It's a hybrid device that combines the high-speed hardware of a switch with the IP-smarts of a router.

This video from Jeremy's IT Lab provides a fantastic explanation.

As the video highlights, an L3 switch can:

• Route between VLANs using Switched Virtual Interfaces (SVIs).
• Form L3 connections to other routing devices using Routed Ports.

Let's look at how we use these two features to build our topology.

SVI Placement and IP Assignment

The SVI is the virtual Layer 3 interface for a VLAN. For every VLAN in your IPAM that needs to communicate with other networks, you will create a corresponding SVI on your L3 distribution switch. The IP address you assign to this SVI is the default gateway from your IPAM document.

Configuration Steps

Configuring an SVI is straightforward, but there are a few critical steps.

The four key steps are:

1. Create the VLAN: vlan [vlan-id]
2. Create the SVI: interface vlan [vlan-id]
3. Assign the IP address: ip address [gateway-ip] [subnet-mask]
4. Enable IP Routing: ip routing (in global configuration mode)

The ip routing command is absolutely crucial. Without it, the device will not build a routing table and will not forward traffic between subnets, even if the SVIs are configured correctly.

This is a common point of failure in real-world deployments. The next video demonstrates not only the SVI configuration but also what happens if you forget this vital command.

Routed Ports: Connecting to the Outside World

SVIs handle routing inside your campus between your VLANs. But how does your campus talk to other networks, like the internet or another site? For this, we use routed ports.

A routed port is a physical switch port that has been converted to operate like a router interface. You do this with the no switchport command in interface configuration mode. Once converted, you can assign an IP address directly to the port, just like on a router. These are typically used for point-to-point links between L3 devices.

For Aperture Logistics, we will use a routed port on our core L3 switch to connect to an edge router that provides internet access. This link will use one of the /30 subnets from our IPAM, like 10.254.0.0/30.

Let's see how this is configured.

The Aperture Logistics L3 Topology

Let's consolidate everything we've learned into a defined topology for Aperture Logistics.

• Design Model: L2 Access / L3 Distribution.
• Core Device: A single (for now) L3 switch, let's call it D1. This switch will be the routing hub.
• SVI Placement: All SVIs for our user, voice, server, and IoT VLANs (from the IPAM) will be configured on D1. D1 will host the default gateway for every internal subnet.
• Access Layer: Access switches (A1, A2, etc.) will be L2 devices. They will connect to D1 via L2 trunk links, carrying all necessary VLANs.
• Internet Connectivity: D1 will connect to an edge router (R1) via a dedicated routed port. This point-to-point link will be addressed with a /30 subnet (e.g., 10.254.0.0/30). D1 will have a default route pointing to R1.

Here's a simple text diagram of our defined topology:

                  +----------------+
                  |  Internet (R1) |
                  | 10.254.0.2/30  |
                  +-------+--------+
                          |
                          | (Routed Port, L3 Link)
                          |
                  +-------+--------+
                  |  D1 (L3 Switch)|
                  | 10.254.0.1/30  |
                  |----------------|
                  | SVI VLAN 110: 10.10.0.1/24
                  | SVI VLAN 80:  10.20.0.1/23
                  | SVI VLAN 90:  10.20.2.1/23
                  | ... (all other SVIs) ...
                  +-------+--------+
                          |
               +----------+----------+
               | (Trunk Link, L2)  | (Trunk Link, L2)
               |                     |
      +--------+-------+     +-------+--------+
      | A1 (L2 Switch) |     | A2 (L2 Switch) |
      +----------------+     +----------------+
           |       |              |       |
       (VLAN 110) (VLAN 80)   (VLAN 110) (VLAN 135)

This design clearly defines our L3 topology, SVI placement on device D1, and IP assignments for both the internal gateways (SVIs) and the external-facing link (routed port).

Conclusion

In this lesson, you have defined a complete Layer 3 routing topology. You moved beyond a simple list of subnets and gateways to architect the logical structure of the network. This is a critical skill for any network professional, bridging the gap between planning and implementation.

Key Takeaways:

• The L2/L3 boundary is a primary design decision. For our campus, we chose a centralized L3 Distribution model.
• Switched Virtual Interfaces (SVIs) are the L3 interfaces for our VLANs. In our design, they are all placed on the central L3 distribution switch.
• The ip routing command is mandatory to enable a multilayer switch to perform routing functions.
• Routed Ports (no switchport) are used to create L3 point-to-point links between routing devices, such as connecting our core switch to the internet edge router.

Preview of the Next Lesson:
We've designed the road system, and now it's time to watch the traffic. In the next lesson, we will trace the packet flow for a host in one VLAN communicating with a host in another VLAN via an L3 switch. This will solidify your understanding of how frames and packets are processed as they move through the architecture you just designed.