trace to serial interface cause extra hop

[seri.ny]

When I trace from router to it own serial interface's ip address, it will reply with two hop to reach interface. Why does it have to go to the next hop interface before come back? This doesn't occur on ethernet interface.

[raymong]

Pings and traceroutes are handled differently by the router when the destination is an ethernet versus a serial interface. When you ping/trace to a serial interface, the packet actually goes out that interface and hits the next hop. This is the reason that you can not usually ping your own serial interface, but you can ping the next hop address.