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TechKnowSurge Catalog Videos Connecting to Router
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Connecting to Router

Covers how to connect a Layer 3 switch to a router to enable internet access beyond inter-VLAN routing, including both routed port and Switch Virtual Interface (SVI) approaches.

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About this video

Once inter-VLAN routing is functioning between VLAN 10 and VLAN 20 through Switch Virtual Interfaces on a Layer 3 switch, there is still a critical gap: traffic cannot yet reach the internet. A router provides that external connectivity, but a properly configured link between the Layer 3 switch and the router must first be established. Best practice calls for placing this uplink on its own isolated network rather than sharing it with existing VLANs, which improves both security and administrative control. A /30 subnet — such as 10.240.0.0/30 — is well suited for this purpose since it provides exactly two usable addresses, one for each endpoint. On the router side, the configuration is straightforward: a physical interface is assigned an IP address and functions as a standard routed port. The Layer 3 switch side offers two valid approaches. The first is to convert one of its physical ports into a routed port, giving it a direct IP address and eliminating any VLAN association on that link. The second is to create a new SVI mapped to a dedicated VLAN, then assign the physical uplink port to that VLAN, allowing the SVI to serve as the logical gateway for that network segment. Both methods are explored in detail, giving network engineers a clear understanding of when and why to choose each option when connecting a Layer 3 switch to an upstream router.

What you'll learn

What's covered

Layer 3 Switch to Router Connectivity

Aligned to

Cisco CCNA
2.1 Configure and verify VLANs (normal range) spanning multiple switches
3.1 Interpret the components of routing table
3.3 Configure and verify IPv4 and IPv6 static routing
3.2 Determine how a router makes a forwarding decision by default

Key terms

Virtual LAN
VLAN
A logical grouping of network devices that behave as if they are on the same network regardless of physical location.
Layer 3
The Network layer of the OSI model, responsible for logical addressing and routing packets between networks.
Router
A network device that forwards data packets between networks based on IP addresses.
Switch Virtual Interface
SVI
A virtual layer 3 interface on a switch associated with a VLAN. On a layer 2 switch a single SVI provides the management IP address; on a layer 3 switch, SVIs also route between VLANs.
Routed Port
A switch port on a multilayer switch that has been configured to operate like a router interface at Layer 3 rather than as a switching port, allowing it to be assigned an IP address and participate in routing protocols. Routed ports are used where a Layer 3 boundary is needed directly on the switch without a separate router.
Inter-VLAN Routing
The process of forwarding traffic between two or more VLANs using a Layer 3 device, which is required because VLANs are separate broadcast domains and cannot communicate at Layer 2 alone. Inter-VLAN routing can be implemented using a physical router with a trunk link (router-on-a-stick), a multilayer switch with SVIs, or routed ports.
Default Gateway
The router that a device uses to send traffic to destinations outside its local network.

Topics

Networking Layer 3 Switching Inter Vlan Routing Switch Virtual Interfaces Routed Ports Vlan Configuration

Transcript

Now at this point we have connectivity between our different VLANs. We set up switch virtual interfaces on a layer 3 switch and now we have connectivity, although we still have to connect to the rest of the world. We have to use the router to get to the rest of the world, so we need connectivity between the layer 3 switch and the router, which creates a little bit of a problem for us.

We set up switch virtual interfaces on our switch, our layer 3 switch, so we can communicate between VLANs, and we actually just specified two. We specified one for VLAN 10 and we specified one for VLAN 20. So now we have connectivity, we are able to route between VLAN 10 and VLAN 20.

Getting to the Outside World

The thing is that there's still a missing piece, and that is, how do we get to the outside world? This router is connected to the internet and we want to gain access to the internet, so somehow we need to get to the outside world. That means that we need to establish this connection between our layer 3 switch and our router, and it needs to be a connection that is on a different local area network. Actually it could be on one of these — it could be on VLAN 20, VLAN 30, VLAN 40, it could be. But what the preference is, so we have more control and more security, is it would be on its own network.

So we actually have to assign a network to 10.240.0.0/30. And I chose a really small network because we just have two interfaces, the router interface and the layer 3 switch interface. So just two interfaces, so I'm just going to do a /30 on here, and that should be good for the purpose of this.

The Router Side

Now on the router side, in both of these cases, and both of my examples that I'm going to give, we're just going to make that strictly a routed port. That's typical: it's a physical port with an IP address on it, and that's it. So in both of these cases, that part of the equation doesn't change. The router is going to be configured with a physical port.

Two Options on the Switch

But what we have the option to do is we have two different options with this layer 3 switch. So one of the options is to turn one of the ports on that side into a routed port, so we could have a routed port on that side that makes a connection between these two devices.

All right, so the other option is we could actually set up a switch virtual interface on here that's going to specifically be a connection between these two devices. So that way, and in that scenario, we would assign it to a VLAN, and then we would assign the port that this switch is connected into — we would assign that port to that VLAN.

So we're going to take a more in-depth, closer look into both those options. But really the gist of this is that we have a router and a switch that need to be connected together. Typically what we do with that is we put it on its own network, and therefore we have to have a connection between there, and we'll have to set up a new SVI.

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