← lab guides · 2026-08-20 · 90m

VLANs + Inter-VLAN Routing on IOS

Trunk, SVI, and the native VLAN gotcha that breaks pings.

topic: routing-switching · tool: cisco-ios, eve-ng · difficulty: beginner

TOPOLOGYFig. — lab diagram

[ LAB TOPOLOGY — add topology.svg to this bundle ]
  [ PC1 ]──[ SW1: vlan10 ]──[ R1 ]──[ FW ]── Internet
              └─ vlan99 MGMT  vlan20 OT (PLC/HMI)

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Objective

You split a two-switch, one-router lab into VLAN 10 (LAB), VLAN 20 (OT), and VLAN 99 (MGMT). You route between VLANs with router-on-a-stick and then check isolation.

Steps

Create VLANs on SW1 and SW2 and assign access ports.
Trunk the link between switches with switchport mode trunk and switchport trunk allowed vlan 10,20,99.
On the router, build subinterfaces with encapsulation dot1Q 10 and assign an IP for each VLAN.
Test with ping across VLANs. Verify with show vlan brief and show ip interface brief.

Gotcha

If you mismatch native VLANs, you drop frames with no error. Set switchport trunk native vlan 99 on both sides, or keep both sides at default 1. I left one side at default and spent 20 minutes chasing it.

Takeaways

Router-on-a-stick versus SVI tradeoffs
Native VLAN changes forwarding, it is not cosmetic
show interfaces trunk tells you the truth

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