Perhaps there is an aspect of the response from Vishwamurti which could be clarified. Using subinterfaces on a router FastEthernet interface configures it to do trunking and to communicate with a switch whose interface connecting to the router is configured as a trunk. Each subinterface of the router interface corresponds to a VLAN in the trunked interface from the switch.
Again adding to the usage of subinterfaces. Apart from it's use in intervlan routing, sub interfaces are also used in Frame-relay networks, to divide partially meshed network into smaller fully meshed sub networks.
Point-to-Point sub interfaces are generally the preferred method in frame-relay networks.
Subinterfaces are also often used for ATM network too.
The key concept of subinterfaces, they allow for multiple logical interfaces off one physical interface. As other posters have described, these logical interfaces would behave much a physical interfaces connected to various networks whether they are Ethernet VLANs, frame-relay or ATM virtual circuits, etc.
A couple of issues using subinterfaces, because they share a physical interface, there are sometimes configuration restrictions compared to what might be done on the physical interface, and since they share a physical interface they also share its bandwidth.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...