Nothing says you have to run IP over an ethernet switch and a group of things that can arp each other - you could very well run something as weird as novell netware or your own custom software stack. The link layer is exactly that, a link layer, it's most often used with IP but not as an absolute. Understanding that the link layer exists underneath and does things independently of the IP network is important.
In an opposite example, you can build an IP network out of two devices that are adjacent to each other over a SDH/SONET network, and there's no Ethernet anywhere. Something as simple as two routers with OC192 interfaces sitting next to each other on a test bench with a fiber patch cable between them, as a BGP adjacency.
