http://zzzmirror.i2p/topics/164-how-to-run-a-router-only-no-clients-at-all
You are providing routing service to the network, but you can't use the network for anything yourself, because you can't send or receive data via I2CP. For extra credit, disable the router console also, by changing clientApp.0.startOnLoad=true to false in clients.config (you can't disable the router console via the router console, for safety).