2.1.2. Layer 2 and Layer 3 Switches
Switches are the workhorses of local networks—fast, reliable, and relatively simple. A Layer 2 switch learns which devices are connected to which ports by watching source MAC addresses, then uses that knowledge to forward frames only where they need to go. Unlike the old hubs that blasted every frame to every port, switches are selective—and that's why modern networks can handle thousands of devices without choking.
But here's the limitation: Layer 2 switches only understand MAC addresses. They have no concept of IP addresses or subnets. If two devices are in different VLANs, a Layer 2 switch can't help them communicate—you'd need a router.
Enter the Layer 3 switch. It does everything a Layer 2 switch does, but it also routes between VLANs using hardware ASICs. Instead of sending inter-VLAN traffic to an external router and back (which adds latency), the L3 switch routes it internally at wire speed. For most campus networks, L3 switches have replaced traditional routers for internal traffic.
| Feature | L2 Switch | L3 Switch |
|---|---|---|
| Forwarding based on | MAC address | MAC and IP address |
| Inter-VLAN routing | No (needs external router) | Yes (built-in, wire speed) |
| Routing protocols | No | Yes (OSPF, EIGRP) |
| Typical use case | Access layer | Distribution/Core |
What happens without enough switching capacity: Network congestion, slow performance, and in severe cases, frame loss. Oversubscribed switches become bottlenecks.
Common Cisco Switch Platforms:
- Catalyst 9200: Access layer, small-medium enterprise
- Catalyst 9300: Stackable access/distribution, very popular
- Catalyst 9400/9500: Modular distribution/core
- Nexus: Data center (high density, low latency)
⚠️ Exam Trap: The term "multilayer switch" means Layer 3 switch. When you see it on the exam, think "switch that can also route."