Configure ISDN on R2 and R6 for DDR as per Figure 7-1.
Advertise ISDN network in Area 0 using clear-text authentication.
Configure the Dialer interface on both routers, and do not use any legacy commands.
Configure DDR Backup using Dialer-Watch on R2 and R6 to come up when “any” interface on R4 goes down.
Identify unique routes learned via R4 to be configured as watched routes. In the absence of a watched route from the routing table, DDR will kick in and bring up the ISDN.
See the following example for R2 and R6 configuration:
<snip from R2 config> hostname r2 ! interface Dialer1 ip address 188.8.131.52 255.255.255.252 encapsulation ppp ip ospf authentication-key 7 05080F1C2243 dialer ...