IPSec VPN between a FortiGate and a Cisco ASA with multiple subnets

When a Cisco ASA unit has multiple subnets configured, multiple phase 2 tunnels must be created on the FortiGate to allocate to each subnet (rather than having multiple subnets on one phase 2 tunnel).

The FortiGate uses the same SPI value to bring up the phase 2 negotiation for all of the subnets, while the Cisco ASA expects different SPI values for each of its configured subnets. Using multiple phase 2 tunnels on the FortiGate creates different SPI values for each subnet.

To configure multiple phase 2 interfaces in route-based mode:

config vpn ipsec phase2-interface
    edit "First subnet"
        set phase1name "VPN to Cisco"
        set src-subnet 192.168.227.253 255.255.255.255
        set dst-subnet 10.142.0.0 255.255.254.0
    next
    edit "Second subnet"
        set phase1name "VPN to Cisco"
        set src-subnet 192.168.227.253 255.255.255.255
        set dst-subnet 10.143.0.0 255.255.254.0
    next
end

To configure multiple phase 2 interfaces in policy-based mode:

config vpn ipsec phase2
    edit "First subnet"
        set phase1name "VPN to Cisco"
        set src-subnet 192.168.227.253 255.255.255.255
        set dst-subnet 10.142.0.0 255.255.254.0
    next
    edit "Second subnet"
        set phase1name "VPN to Cisco"
        set src-subnet 192.168.227.253 255.255.255.255
        set dst-subnet 10.143.0.0 255.255.254.0
    next
end