Textbooks like to go through the steps of what happens to a packet as it goes from client to server through a single router. This always makes sense...The client application payload gets handed down the TCP/IP stack to layer 3, layer 2, etc., then back up at the other end.
In the scenario, the client sends the packet to the default gateway which terminates on the router's Ethernet interface. The router sees the MAC address as its own, so it inspects the destination IP address and looks it up in the routing table to find out the outgoing router interface to send the packet. Here the scenario always shows what happens when the outgoing is an Ethernet interface...it ARPs the MAC address of the destination server because Ethernet NEEDS to know the destination MAC in order to build the frame. This makes perfect sense.
What happens though, if it is to go to an outbound serial interface? Whereas Ethernet requires a HARDWARE MAC address to send the frame... what does a SERIAL output buffer look for as a "hardware destination address" in order to package the layer 2 frame and send it on its way? I'm thinking we could be sending data via HDLC, PPP, Frame Relay(point-to-point and multipoint). For HDLC, PPP will it always just be a broadcast address? For Frame, will it use local DLCI?
