what happens internally in a router when lan is configured?

Set up internet connection redundancy cisco router 3640?

• i have a 3640 router which has two interfaces connected to different ISP's interface f0/0 it's connected to a cable modem configured with the static ip provided by ISP interface f0/1 it's connected to a DSL modem/router IP address via DHCP interface f2/0 is may LAN interface i hve an ACL permitting access to my lan: access-list 1 permit 192.168.110.0 0.0.0.255 my nat configuration is ip nat inside source list 1 interface F0/0 overload int f0/0 ip nat outside int f02/0 ip nat inside i have one static route configured as follows: ip route 0.0.0.0 0.0.0.0 (ip of default gateway ISP) cable modem I would like to configure my router to switch over the back up line (DSL) when the cable link goes down .... any help will be appreciated thanks

• Answer:

  If you're looking for physical redundancy only, make a second static route pointing to the second ISP, and give it a higher administrative distance, such as "ip route 0.0.0.0 0.0.0.0 (ip of second ISP default gateway) 2". Since 2 is greater than the default AD of a static route (1), it will only take affect if physical connection on ISP 1's link goes down. You will also need to redefine that static nat statement to use a pool instead of just the primary outbound interface, and you'll need to define Fa0/1 as an outside interface (ip nat outside). Now, that will only switch over dynamically if the connection on Fa0/0 goes physically down. To switch over in the event of ISP network failure (as in, a problem further upstream in the connection than the modem), you may need to configure BGP routing with your ISPs. One far more advanced option is EEM (Embedded Event Manager), with which you could write a script to reconfigure the ports based on (for example) interface traffic going outside of the normal network usage baseline suggesting the connection is down. I've written similar with a good deal of success, but that is dependent on IOS version, etc.