I posed the philosophical question on Twitter the other day asking if single trunk links should be in an EtherChannel bundle just in case you need to expand later. I didn’t really expect an answer, but the ever-verbose @WannabeCCIE pointed out (in not so many words) that you should watch your IDBs. What is that?
That’s an interface descriptor block. I admit that I’m not intimately familiar with them, bu they’re data structs in IOS used to keep track of the interfaces on that device. They come in two flavors - hardware and software. HWIDBs usually represent a physical interface but they also represent tunnels, SVIs, PortChannels, subinterfaces, and any other virtual interface that you can configure. The SWIDBs represent the layer-2 encapsulation of each HWIDB, so you’ll see entries talking about Ethernet, HDLC, PPP, etc. That means that every interface you have on a router consumes two IDBs (there are always exceptions). That’s important because each platform and IOS version combination has a limit to the number IDBs that device supports.