Chapter 14. Making the network scalable by connecting routers and switches together
Right now, Switch1 routes IP traffic between two subnets: 172.31.60.0/24 in VLAN 600 and 172.31.70.0/24 in VLAN 700. This is a common and perfectly acceptable configuration. But there are times when you’ll want to route inter-VLAN traffic using an additional device: a router.
In one case, you may run into a network that just doesn’t have any routers or layer-3 switches. It’s unlikely that such a network would be using VLANs to begin with, because there wouldn’t already be any way to route between them. Hence, if you walk into an organization that needed VLANs set up, but all they have are some layer-2 switches and a router, then the only way to route traffic between VLANs is to use a router.
This specific use case for a router is becoming less and less common as layer-3 switches become more ubiquitous. But organizations of all sizes use routers heavily for other critical functions, including network connectivity between offices and connecting IP phones to the public switched telephone network (PSTN). That’s why, as a Cisco network administrator, it’s important that you have a clear and correct understanding of how these devices function.