§ 02 — Network Administration
VLANs separate a network into different broadcast domains. That separation improves organization and security, but it also means devices in different VLANs cannot communicate at Layer 2 alone. Inter-VLAN routing provides Layer 3 forwarding between VLANs.
By the end of this module, you should be able to:
Devices in the same VLAN can communicate through switching. Devices in different VLANs need a router or Layer 3 switch to forward traffic between their IP networks.
For example:
192.168.10.0/24.192.168.20.0/24.A router uses a separate physical interface for each VLAN. Each router interface connects to a switch access port in a different VLAN. This works but does not scale well because each VLAN needs a physical router interface.
A router uses one physical interface configured with multiple logical subinterfaces. The switch port connected to the router is a trunk. Each subinterface is assigned to a VLAN using 802.1Q encapsulation and acts as that VLAN's default gateway.
A Layer 3 switch uses SVIs as VLAN gateways and routes internally between them. This is common in larger LANs because it is faster and more scalable than sending all inter-VLAN traffic to an external router.
In router-on-a-stick, the router receives tagged frames from a trunk link. The tag identifies the VLAN. The router processes traffic through the matching subinterface and routes it to the destination VLAN.
Each subinterface needs:
06 — Activity: Configure Router-on-a-StickPro reviewer — not included in this printout.
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A Layer 3 switch can route between VLANs using SVIs. Each VLAN interface receives an IP address and becomes the default gateway for hosts in that VLAN. Routing must be enabled with ip routing.
08 — Activity: Configure Inter-VLAN Routing on a Layer 3 SwitchPro reviewer — not included in this printout.
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Common issues include:
ip routing is missing on a Layer 3 switch.10 — Activity: Troubleshooting ChecklistPro reviewer — not included in this printout.
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Inter-VLAN routing allows communication between separate VLANs. Router-on-a-stick uses router subinterfaces over a trunk link. A Layer 3 switch uses SVIs and internal routing. Most failures come from mismatched VLANs, trunk problems, missing gateways, or incorrect interface configuration.
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Lesson 5: STP Concepts→←Previous: Lesson 3: VLANs