This isn't "switch vs. router" — it's a question about one single switch's own capability. Can it move traffic between VLANs itself, or does every inter-VLAN packet have to leave the switch and come back?
A Layer 2 switch forwards frames using nothing but MAC addresses, entirely inside one VLAN — it has no concept of a subnet and cannot move traffic between VLANs at all without help from an external router. A Layer 3 switch (multilayer switch) is the same switching hardware with one thing added: an IP-routing engine built directly into the switch's own forwarding silicon, so it can route between VLANs internally, at switching speed, without ever sending the packet out to a separate router box. This is a different distinction from "switching vs. routing" as a contrast between two device types — this is about what a single device can do on its own once you add routing capability to its own hardware.
Give a Layer 2-only switch two VLANs — say VLAN 10 and VLAN 20 — and it will happily forward frames within each one all day, using its MAC address table, at line rate. But ask a device on VLAN 10 to reach a device on VLAN 20, and the switch simply has no mechanism for it. It doesn't know what an IP address is, let alone which VLAN is "closer" to the destination. The traditional fix is router-on-a-stick: run a single trunk link from the switch up to an external router, configure a subinterface on that router for each VLAN, and let the router do the actual inter-VLAN routing decision. The packet has to physically leave the switch, get routed by the external device, and come back down the very same trunk link before it can reach the other VLAN.
That works, but it's architecturally expensive: every single inter-VLAN packet, no matter how small, makes a round trip up to the router and back — consuming trunk bandwidth twice and adding a real, measurable extra hop of latency, even though the two VLANs might be plugged into the exact same physical switch chassis three feet apart.
A Layer 3 switch keeps the same physical layout — VLAN 10 and VLAN 20 plugged into the same box — but adds a Switched Virtual Interface (SVI) for each VLAN and an IP routing table, both implemented directly in the switch's own forwarding hardware (its ASIC), not bolted on as a CPU-based software path. When a frame destined for a different VLAN arrives, the switch's hardware pipeline does the Layer 3 lookup itself, right there, in the same silicon that already does the Layer 2 MAC lookup — at the same wire-speed forwarding rate. The packet never leaves the switch. There is no external router, no trunk link consumed twice, no extra hop at all. This is exactly why campus and data-center network designs put Layer 3 switches at the distribution and core layers: inter-VLAN routing gets handled locally, right where the traffic already is, instead of being backhauled to a router every single time.
A traditional router forwards packets by running a route lookup in general-purpose CPU software — flexible, but comparatively slow, and it's exactly why router-on-a-stick designs add a real, measurable hop of latency. A Layer 3 switch instead programs its IP routing table directly into the switch's ASIC — the same purpose-built forwarding hardware that already handles Layer 2 MAC lookups at line rate. Once a route is programmed into that hardware, routing between VLAN interfaces happens at the identical forwarding speed as switching within one VLAN — there is no software detour and no CPU bottleneck for that traffic. That's the entire reason distribution and core-layer switches in campus and data-center designs are built as Layer 3 switches: inter-VLAN routing gets handled locally, at hardware speed, right where most of that traffic already needs to go, instead of every packet making a trip out to a separate router and back.
Not quite — and the difference matters for performance, not just terminology. A switch physically sitting next to a separate router (or a router plugged into it via a single trunk cable, as in router-on-a-stick) still has to send every inter-VLAN packet out to that router and back — the two devices remain architecturally separate, connected by a link with finite bandwidth and real added latency. A genuine Layer 3 switch is different in kind: its IP routing capability is implemented directly in the switch's own ASIC, the same purpose-built hardware that performs its Layer 2 MAC address lookups, so routing between VLANs happens at the same wire speed as switching within one — with no cable to cross, no separate device, and no CPU-software route lookup in the path. "Layer 3 switch" specifically means routing implemented in switch hardware at wire speed — not two devices bundled together, and not a slower CPU-based routing path grafted onto a switch.That last caveat matters too: some low-end "smart switches" marketed as Layer 3-capable actually route in software on a general-purpose CPU rather than in ASIC — they get the inter-VLAN routing feature, but not the wire-speed performance that defines a true Layer 3 switch at the distribution or core layer.
Explains the difference between a pure Layer 2 switch (forwards frames by MAC address within a single VLAN and cannot reach a different VLAN without an external router) and a Layer 3 switch / multilayer switch (adds an IP-routing engine directly into the switch's own ASIC, so it routes between VLANs internally, at wire speed, with no external router hop).
A Layer 2-only switch builds and uses a MAC address table exactly as any switch does, forwarding frames within a single VLAN / broadcast domain at line rate. It has no concept of an IP address, a subnet, or a routing table, so it has zero mechanism for moving a frame from one VLAN to another. Any traffic that needs to cross from VLAN 10 to VLAN 20 must leave the switch entirely and be handled by a separate Layer 3 device — commonly a router connected via a single trunk link in a router-on-a-stick configuration, with a subinterface on the router dedicated to each VLAN.
A Layer 3 switch (multilayer switch) keeps the same Layer 2 forwarding behavior but adds an onboard routing engine: a Switched Virtual Interface (SVI) for each VLAN and an IP routing table, both implemented directly in the switch's forwarding ASIC rather than a general-purpose CPU. When a frame's destination is a different VLAN, the switch performs the Layer 3 lookup itself, in the same hardware pipeline that already performs Layer 2 MAC lookups, at the same wire-speed forwarding rate — the packet never has to leave the switch to reach another VLAN.
In a router-on-a-stick design, every single inter-VLAN packet crosses the trunk link between switch and router twice — once on the way to the router, once on the way back — and takes a full router hop's worth of latency each time, even for two devices plugged into the same physical switch. A Layer 3 switch eliminates that round trip entirely: routing happens inside the switch's own ASIC, at the same speed as switching, with no external device and no extra hop. This is exactly why campus and data-center network designs place Layer 3 switches at the distribution and core layers, handling inter-VLAN routing locally instead of backhauling it to a central router.
"Layer 3 switch" is often misread as simply "a switch and a router combined in one chassis." That's not the defining characteristic — a switch sitting next to a separate router, or connected to one via router-on-a-stick, is still two architecturally separate devices linked by a cable with real added latency. A true Layer 3 switch specifically implements IP routing in the switch's own ASIC hardware, at wire speed, in the same forwarding pipeline as its Layer 2 lookups — which is why it's architecturally different from, and much faster than, a switch that merely has a router attached to it. Some low-end "Layer 3-capable" smart switches route in CPU software rather than ASIC hardware; they gain the inter-VLAN routing feature but not the wire-speed performance that defines a genuine Layer 3 switch.
No — that framing misses what actually makes it fast. The defining feature of a Layer 3 switch is that IP routing is implemented directly in the switch's own ASIC, the same hardware that performs Layer 2 MAC lookups, so routing between VLANs happens at wire speed. A switch physically paired with a separate router box is architecturally different and slower, because traffic still has to cross a real link to reach the router and come back.
Router-on-a-stick is a design where a Layer 2-only switch is connected to an external router over a single trunk link, with a subinterface configured on the router for each VLAN. Every inter-VLAN packet has to travel up that trunk link to the router, get routed there, and travel back down the same link — consuming the link's bandwidth twice and adding the router's own processing delay, even for two devices on the same switch chassis.
Yes. A Layer 3 switch performs ordinary Layer 2 switching (MAC address table, forwarding within a VLAN) exactly as any switch does. What it adds is a second, independent capability — an IP routing table and Switched Virtual Interfaces (SVIs) — implemented in the same ASIC, used specifically when traffic needs to move between VLANs.
Because where the lookup happens determines the speed. A CPU-based software route lookup (as in a traditional router, or a low-end "smart switch" marketed as Layer 3-capable) is comparatively slow and can become a bottleneck under load. A true Layer 3 switch programs its routes into purpose-built forwarding silicon, so the IP lookup runs at the same line rate as the MAC lookup — there's no software detour for that traffic.
Because that's where inter-VLAN traffic naturally converges — the distribution/core layer already aggregates traffic from multiple access-layer switches serving different VLANs. Putting the routing engine there, in hardware, lets that traffic get routed locally at wire speed instead of being backhauled out to a separate router and back for every single inter-VLAN packet.
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