ISP link redundancy

[Anne]

Hi there, We are a small ISP and we use Fortigate 310B' s in vdom mode to provide Internet connectivity to our customers. We use BGP peering (one peer for inbound and peer for ISP connectivity) which works fine. One of the customers has asked for iSP resiliency. So what I proposed was to add a sub-interface on the ISP side and create another linket to another ISP router and use BGP for advertising the routes. How can I achieve Link Redundancy? (whats the best way to do it) How can I configure BGP policy on the firewall to prepend outbound advertisements? (what command) Thanks in advance Anne