We have three locations connected to each other and having two 2 Mbps lease lines and two ISDN lines making a triangle.
Each site is connected with other two sites with one lease line (2 Mbps) and one ISDN line. At one site we have two routers 2611 and 2503. The 2611 router contains two lease lines (2 Mbps) and 2503 router contains two ISDN lines (2 Mbps) both routers have the same IP address. So when both the lease lines gets down we have to shut down 2611 router and manually up the 2503 router and fire the ISDN line manually. I want both the router up all the time (7*24) and in case if both the lease lines down gets down isdn line should gets fire automatically. What type of configuration should I use? I'm currently using an EIGRP network. You might need to change your IP addressing scheme and hardware configuration on these two routers a little bit so that each router can have a serial interface and an ISDN interface. The serial interfaces need to be in same IP subnet and you can give IP unnumbered to ISDN interfaces or of loop-back interface. Now you can have a leased line and an ISDN connection to each of these routers. Now what you need to do is under ISDN interface you need to track serial interface so that as soon as it goes down ISDN will kick up based on interesting traffic. On your LAN side you can put both the Ethernet interfaces in HSRP so that on your LAN you don't have to do any configuration once a link goes down.
So in short:
Have a serial and an ISDN on each router.
Track serial interface from ISDN interface
Run HSRP on Ethernet interface.
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