I hope someone can offer a crystal clear explanation (or resource) for better understanding this...

Say I'm inside my router, and my router has a serial interface connected to a frame relay circuit. I CANNOT ping this serial interface (from inside the router), BUT I CAN ping this serial inteface from a PC on a directly connected Ethernet segment OR from a router across the WAN. What's going on?

I know the fix is to create an IP-to-X.121 address mapping. But could you please explain exactly what's happening to the ping before AND after the mapping, including where the ping is going? I noticed after adding the IP-to-X.121 mapping, a traceroute to my serial interface includes a hop across the frame network. ???

So why does the ping work from the Ethernet segment without any explicit Layer3-to-Layer2 mapping?

Does this apply to X.25 circuits, too?

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Well I am no expert, but when you map IP-to-x.121 I believe you are mapping destination IP to an interface. Have you tried either IP ADDRESS nnn.nnn.nnn.nnn mmm.mmm.mmm.mmm on your X.121 serial interface, but then again you probably did that.

The reason after you created the map your trace includes the frame hop is because your router does not know this local address (which is strange, if you assigned address to the interface)but your destination connection knows about your IP. Check on the interface whater or not loopback machanism was disabled as a default for the serial interface.
When you check routing table does your interface show with the IP address?

To save IP address you can use IP UNNUMBERED on serial with mapping to one of your ethernet int's or just create LOOPBACK INT only usefull with certain protocols logic such as OSPF to help maintain router's ID.

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I am not sure about X.25, but the reason for that in FR is simply that router doesn't dynamically map the logical address (IP) to the physical address (DLCI), and you will need to explicitly map it in the router.

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