Configure ISDN on R3 and R5 using PPP. R3 and R5 should be able to ping each other across the ISDN link.
Make sure that your routing protocol is passed across the ISDN link only when the connectivity is established.
The ISDN link should only pass a routing protocol if R5-fa0/0 is down.
Configure R3 to override specific cause codes that are sent to the switch in case of a disconnection. Should this event occur, ensure that R3 uses "Channel-Not-Available" code message.
