At first it may seem odd—a router is able to switch—but on the other hand it’s quite common for a switch to be able to route. So what’s the difference between a switch that’s able to route and a router that’s able to switch? Is this merely a philosophical discussion, or is there something more to it?
Traditionally, switches are designed to handle only a single Layer 2 network. A Layer 2 network is simply just a collection of one or more broadcast domains. Within the constraint of a single Layer 2 network, a switch makes sense. It’s able to flood, filter, and forward traffic for 4,094 VLANs without a problem.
The problem becomes interesting as the network requirements grow, such as having to provide Ethernet services to multiple Layer 2 networks. Let’s take the scenario to the next level and think about adding multiple Layer 3 networks so the requirement is to support multiple Layer 2 and Layer 3 networks on the same physical interface that has overlapping VLAN IDs, MAC addresses, and IP addresses. This challenge becomes even more interesting as you think about how to move data between these different Layer 2 and Layer 3 networks.
There’s no distinction between the terms “bridging” and “switching,” and they are used interchangeably in this book.
It’s always helpful to see an illustration, so take a moment with Figure 2-1.
Figure 2-1. Traditional switch compared ...