Sonicwall and WAN Failover

Hello, I have 2 Sonicwall's an NSA 3500 and an NSA 4600, I also have 2 internet circuits one is 20Mb and the 2nd 100Mb, I know I can't do Sonicwall failover but would like to set it up so we can have some level of redundancy (even if we have to move a cable), just wondering how others are accomplishing this - the new 100Mb fiber circuit can be handed off as fiber or Cat-6
the 20Mb circuit terminates at a Cisco 2921/K9 router and drops of to ethernet. I know the NSA 4600 has some cards available where we could hand fiber off to it - not sure on the 3500. The intention is to keep both circuits up the 20 for utility functions and the 100 for corporate internet etc.. It would be great if the 100Mb went down to switch it somehow or have some kind of box in front of the firewall that could do this and provide for failover.  
Any ideas would be greatly appreciated.
Thanks for any help
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You can connect both ISP circuits to the same Sonicwall and configure failover as follows: 

You should also be able to configure HA with the two as follows:

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Unfortunately to configure two Sonicwalls in a HA pair, they both need to be the same model, so you would need to purchase an additional NSA 3500 or NSA 4600.

If you do purchase an additional unit, it would be best to connect a switch between the WAN connection handoffs and each firewall. This allows the switch to supply both your 20Mb and 100Mb connections to both firewalls, allowing the Sonicwalls to automatically failover if needed.

You could also manipulate the routing tables to direct your "utility functions" and "corporate internet" traffic out to their correct WAN connections. This configuration will automatically carry across when one Sonicwall fails over to the other.
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