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 only when the ISDN link is up that it will pass routing protocol across the link.
The ISDN link should pass routing protocol only when R5-s0/1 and/or R5-fa0/0 and/or R5-fa0/1 is down. Consider that the routing protocol can pass through the ISDN link when topology changes happen.
