PlantUML DiagramSVIs on a Layer 3 Switch
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| Metric | Router-on-a-stick | Layer 3 switch (SVI) |
|---|---|---|
| Throughput | Limited by one uplink | Wire speed across all ports |
| Latency | Higher — software routing | ASIC-based, ~microseconds |
| Single point of failure | The router | None if redundant L3 switches |
| Physical ports needed | 1 per routed link | None — SVIs are logical |
| Scales to | Small/medium networks | Any size |
| Cost | Reuse existing router | Requires L3-capable switch |
Router-on-a-stick was a clever workaround for hardware limitations. Layer 3 switches made it obsolete. A modern access switch with routing capability can route between its VLANs at line rate, in hardware, without any external device.
A Switch Virtual Interface is a logical routed interface associated with a VLAN. Every VLAN can have an SVI — one IP address, one subnet, acting as the gateway for hosts in that VLAN. Because it's internal to the switch, routing between SVIs happens on the switch's ASIC at wire speed.
The SVIs are virtual — no physical cable connects them. But they function as gateways for their VLANs. When a host in VLAN 10 sends to a host in VLAN 20, the switch receives the frame, determines the destination is in VLAN 20, rewrites the frame headers, and forwards. All internally.
That `ip routing` command is the single most common gotcha. You can configure SVIs, assign IPs, verify everything looks right, and still have no inter-VLAN routing — because routing is globally disabled. Enabling it with one command makes it all work.
A small business has four VLANs, ~200 users total, and a single access switch. Inter-VLAN traffic is moderate. They have a spare Cisco router but no L3-capable switch. What's the appropriate design?
Select the safest next action.
On a Layer 3 switch, configure two SVIs and test inter-VLAN routing. Then disable `ip routing` and re-test. The failure mode is instructive — everything looks configured, but nothing routes.
Finish the lesson once you have worked through the material. This awards ★ 50 XP.