There are many solutions to achieve redundancy on the first hop (Router). Normally one would use two routers which make use of a FHRP (First Hop Redundancy Protocol) like HSRP (Cisco-Propretary) or VRRP (open standard). A virtual ip address would be assigned to one of two routers and in case of failure a failover would occur and the standby router would take over the VIP.

Since in your case you have a firewall and not a router your solutions will differ. You only have one firewall in your disposal and want to connect it redundant to both your switches.

Many firewall vendors offer a "redundant" interface option. A secondary link may be connected to another switch and in case of the primary link going down a failover to the backup link would occur.

You may want to check your software version, FortiGate supports this feature http://docs-legacy.fortinet.com/fos50hlp/50/index.html#page/FortiOS%205.0%20Help/clustering.082.27.html

There are many solutions to achieve redundancy on the first hop (Router). Normally one would use two routers which make use of a FHRP (First Hop Redundancy Protocol) like HSRP (Cisco-Propretary) or VRRP (open standard). A virtual ip address would be assigned to one of two routers and in case of failure a failover would occur and the standby router would take over the VIP.

Since in your case you have a firewall and not a router your solutions will differ. You only have one firewall in your disposal and want to connect it redundant to both your switches.

Many firewall vendors offer a "redundant" interface option. A secondary link may be connected to another switch and in case of the primary link going down a failover to the backup link would occur.

You may want to check your software version, FortiGate supports this feature http://docs-legacy.fortinet.com/fos50hlp/50/index.html#page/FortiOS%205.0%20Help/interfaces.100.12.html