Configure the ISDN interfaces on R1 and R4 as follows:
■ When R1's S0 goes down, R1 should place an outgoing call to R4.
■ R4 cannot call R1 under any circumstance.
■ Use PPP encapsulation for one B channel, HDLC on the second, and the strongest authentication available where applicable.
■ Never bring up more than one B channel, to ensure that costs are kept to a minimum; you can use the interface configured with PPP for this purpose.
■ When the Frame Relay link is restored, bring down the ISDN link after 25 minutes.
■ When the ISDN link is active, all routers must be able to ping and Telnet the local ISDN interfaces on R1 and R4.
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