I have Cisco 2811 router with four T1 v2 wics. Two of these wics are used for private lines. The other two are used for T1s to the Internet. Currently, only one is in use. We just purchased an additional T1 from a different ISP with another set of public IPs. I would like to load balance these lines to the Internet and also provide failover capabilities incase one goes down. I am not sure how to configure this setup. Do my ISPs have to do anything on there end? What should be done on my router?

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Your ISPs do not have to do anything. To load balance, the first step is to create an access list that will divide your network into two. So with access-list you need to define half the IP addresses on one link and therest on another link.

Lets say you network is 172.16.128.0/24. "Access-list 1 permit ip 10.172.16.128.0 0.0.0.254" will permit only even IPs. So now you have two subnets. You might have to change this per your requirements and IPs. Now you can creat a route map.

Route map 10 ISP1_primary
Match access-list 1
Set interface ISP1_interface

Route map 20 ISP1_primary
Match access-list 1
Set interface ISP2_interface

Similarly you need to create one more route map for second ISP.

Route map 10 ISP2_primary
Match access-list 2
Set interface ISP2_interface

Route map 20 ISP1_primary
Match access-list 2
Set interface ISP1_interface

Access-list two is another access-list for matching the networks. You need to devise your own ways to divide the network. One more option is to add floating static routes on the routers.

This was first published in August 2005

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