Chapter 2 Advanced Routing for FortiOS 5.0 : Border Gateway Protocol (BGP) : Dual-homed BGP example : Testing this configuration : Verifying the dual-homed side of the configuration
  
Verifying the dual-homed side of the configuration
Since there are two connections to the internet in this example, theoretically you can pull the plug on one of the ISP connections, and all traffic will go through the other connection. Alternately, you may choose to remove a default route to one ISP, remove that ISP’s neighbor settings, or change the weightings to prefer other other ISP. These alternate ways to test dual-homing do not change physical cabling, which may be preferred in some situations.
If this does not work as expected, things to check include:
default static routes — if these are wrong or don’t exist, the traffic can’t get out.
BGP neighbor information — If the ISP router information is incorrect, the FortiGate unit won’t be able to talk to it.