Layer 3 Switch (Multilayer Switch)

Definition

A Layer 3 Switch (Multilayer Switch) is a network device that works as both a Layer 2 Switch and a Layer 3 Router. It switches data within the same VLAN and also routes data between different VLANs at very high speed.

Its role is to let devices in different departments (VLANs) communicate without sending traffic to an external router.

What Problem It Solves

Before Layer 3 switches:

• Layer 2 Switch → Connects devices inside the same VLAN.
• Router → Connects different VLANs using Router-on-a-Stick.

As networks grew, every inter-VLAN packet had to travel:

PC → Switch → Router → Switch → Destination

This created a bottleneck because one router interface handled all VLAN traffic.

A Layer 3 Switch performs routing internally:

PC → Layer 3 Switch → Destination

No external router is needed for internal VLAN communication.

What Happens If Not Used

Without a Layer 3 Switch:

• Different VLANs cannot communicate by themselves.
• You must configure Router-on-a-Stick.
• Router becomes a traffic bottleneck.
• Performance decreases as the network grows.

Easy Wording

A Layer 3 Switch is a switch that can also act like a router, allowing different VLANs to communicate much faster.

Layman Example

Imagine a shopping mall.

• Layer 2 Switch: People can move freely inside one floor.
• Router: To go to another floor, everyone must use one elevator.
• Layer 3 Switch: Every floor has its own escalator directly connecting all floors, so people move much faster.

Technical Example

Before Layer 3 Switch (Router-on-a-Stick)

PC-A (VLAN10) │ ▼ Layer 2 Switch │ (Trunk) ▼ Router (Subinterface VLAN10 & VLAN20) │ ▼ Layer 2 Switch │ ▼ PC-B (VLAN20)

Flow:

• PC-A sends packet.
• Layer 2 Switch forwards it to the router.
• Router routes between VLANs.
• Packet returns to the switch.
• Switch forwards it to PC-B.

After Layer 3 Switch

PC-A │ ▼ Layer 3 Switch (SVI VLAN10 & VLAN20) │ ▼ PC-B

Flow:

• PC-A sends packet.
• Layer 3 Switch receives it.
• Switch routes internally using the destination IP.
• Packet is immediately forwarded to VLAN20.
• PC-B receives it.

No external router is involved.

Limitation

A Layer 3 Switch is excellent for routing inside the local network (LAN).

However, it usually does not replace an edge router because it lacks or is less suited for features such as:

• Internet/WAN connectivity
• NAT (Network Address Translation)
• VPN termination
• ISP-specific routing and security services