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I currently manage several office locations which each have two internet connections, a DSL line and some bonded T1 lines. Currently, the DSL line is used for the internet and the bonded T1 lines are used for site-to-site IPSEC VPN tunnels for the internal network. This is all handled by Cisco 2800 series routers at each location which handle the VPN tunnels and splitting the traffic between the DSL lines and T1 lines.

The T1 site-to-site VPN tunnels are going to be replaced by a MPLS network. The MPLS provider is going to provide CISCO 1841 routers to act as the gateway, which my 2800s will connect to. I am going to keep the DSL lines for internet traffic. So my 2800 will connect to both the DSL line and the MPLS network.

Is there a way I could set up 2800s to use the VPN tunnels going over the DSL lines as a fail over for internal traffic if the lines go down. Or, even better, is there a way I could those VPN tunnels going over the DSL lines to fail over if the MPLS line goes down or if the MPLS line's bandwidth is currently maxed out? What would that entail?

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This would depend on what your ISP provides you with in the form of routing. As long as your 2800 and the ISP's 1841 are exchanging routes and the 1841 is exchanging routes with the provider edge, it should "just" be a matter of configuring dynamic routing or floating statics across VPN links over the DSL link(s).

So that in the event of one (or more) sites losing MPLS connectivity, the routes across the DSL network would be the most preferred, but with an active MPLS link, the MPLS network would be preferred.

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You should be able to have the VPN over the DSL act as backup link to your MPLS and vice versa pretty easily using floating static routes.

This article gets into some additional details of using tracking interfaces and such that is a more advanced solution: http://tcpmag.com/qanda/article.asp?EditorialsID=394

Using the DSL/VPN if the MPLS circuit is maxed out could be significantly harder. What determines maxed out? When the traffic flips over and the demand on the on the MPLS goes down, does the traffic then flip back? This sort of thing can be difficult to engineer in such a way that you don't end up causing more problems than you solve.

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