I have two leased lines from two different ISP, I need to configure both these lines in one router (2511) which...
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has two serial and one Ethernet interface. Is this possible, if so what I should I need to do so that always there will be fallback line to have redundancy in my network.
My aim is to have a backup line so that when one line goes down the traffic should go through other leased line.
The answer to your question is floating static route. You can configure two static routes, one pointing to the first leased line and the other pointing to the second leased line. Suppose these lines are connected to serial 0 and serial 1 interfaces. Then your static routes would look like
Ip route 0.0.0.0 0.0.0.0 serial 0 Ip route 0.0.0.0 0.0.0.0 serial 1
This will load balance the traffic between the two lines. If you don't want load balancing, and want second line purely as backup, then your configuration would look like –
Ip route 0.0.0.0 0.0.0.0 serial0 Ip route 0.0.0.0 0.0.0.0 serial1 150
The 150 command in the second route here tells the router that the administrative distance for the second route is 150 (for normal static route it is 1). So the second route has a less preference than the first route, and should only be used when the first route fails.
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