Configuring a 'router on a stick' or a 'one-armed router'

Designing High Performance Campus Intranets [Campus Switching Solution] - Cisco Systems: "Router X is typically referred to as a 'router on a stick' or a 'one-armed router.' More routers can be used to distribute the load, and each router attaches to several or all VLANs. Traffic between workgroups must traverse the campus in the source VLAN to a port on the gateway router, then back out into the destination VLAN."

Why would you want to do this? Imagine that you have 2 subnets that you need to route between (10.1.1.x and 10.1.2.x). Normally, you would place on interface in each subnet and everything would be fine. But what if the router you are using only has one free interface? In this case, using one-armed routing would be one solution.

To do this, you set up the interface on the router and the interface on the switch that it is conencted to as trunking ports. You can now route between both subnetsvia the single routed interface.

Things to remember:
1. You need to ensure that the trunking protocol between the router and the switch is the same.
2. The older Cisco 1900 switches only support ISL.
3. The newer Cisco 2900 switches only support 802.1q
4. Cisco 1700 routers only support 802.1q