ISDN configuration as a Backup

Oops! Disregard previous.

RouterA:
isdn switch-type (whatever it is)
username RotuerB password abcdef
int e0
 ip address 192.168.1.1 255.255.255.0
int bri0
 ip address 10.1.1.1 255.255.255.0
 encap ppp
 ppp authentication chap
 isdn spid1 40455512110101
 isdn spid2 40455512120101
 dialer-group 1
 dialer map ip 10.1.1.2 name RouterB 4045551221
ip route 192.168.2.0 255.255.255.0 10.1.1.2
dialer-list 1 ip permit

RouterB:
isdn switch-type (whatever it is)
username RotuerA password abcdef
int e0
 ip address 192.168.2.1 255.255.255.0
int bri0
 ip address 10.1.1.2 255.255.255.0
 encap ppp
 ppp authentication chap
 isdn spid1 40455512210101
 isdn spid2 40455512220101
 dialer-group 1
 dialer map ip 10.1.1.1 name RouterA 4045551211
ip route 192.168.1.0 255.255.255.0 10.1.1.1
dialer-list 1 ip permit

-Don

You should probably use the ISDN solely for backup purposes or else the charges will be outrageous.  

I would recommend using the backup command in the interface you need to have the backup for.

interface Serial 0
backup interface BRI0
backup delay 30 30
!
interface BRI0
(continue with config above)

This way the interface is not active until Serial0 interface has been down for 30 seconds.  Once Serial0 is back up for 30 seconds, ISDN (bri0) will shutdown.)

Are you using any routing protocols?

Many thanks for all....


No we are  not using any routing protocol...


This ISDN internet backup link we are getting from the ISP. In that case can i use the configuration details which mentioned above.


i heard that ISP will be providing one set of username and a dialing number. Once the primary link in serial 0 goes down this ISDN should dial and get connected to internet.

What are the command need to given the router


Thanks in advance

Some of the answers above have most of the information but I found a doc on CCO that has a nice network diagram and should be very easy to follow along with if you are unfamiliar with bacukp scenarios.  It uses the "backup interface" commandlike dgroscost recommended.  The only thing that will be different in your configuration is that you will not be using OSPF.  In your case you will need to use floating static routes.  You will have two default routes; assuming your outbound interface is Serial0 it would look like this:

ip route 0.0.0.0 0.0.0.0 serial0
ip route 0.0.0.0 0.0.0.0 dialer0 254

The 254 at the end of the second route is a "preference" where lower values are more preferred.  constructing routes that point to the interface are important because the route will be removed from the routing table when the interface goes down.  So what we are acomplishing is when the S0 interface goes down, the second route will be installed in the FIB using the dialer0 interface.

You can further fine tune the configuration by adjusting the "dialer-list 1 protocol ip permit".  This command defines interesting traffic for the interface.  It depends very much on what type of traffic you are sending so if you find that you would like more of an explanation of how it works just let me know.  Good luck!

seatlh could you post that link please.

My bad.  Sorry about that.

http://www.cisco.com/en/US/tech/tk801/tk133/technologies_configuration_example09186a008009455d.shtml