IPSEC negotiation on demand

[Matthew Closson]

Rather than have isakmpd bring up all tunnels when the daemon starts up, 
is there a way to have it bring up the tunnels on demand?  For example.

host_a  ---->  router_b <------------> router_c <----- host_d
Is there a way to setup isakmpd so that if host_a tries to send a packet 
to host_d, router_b will start IPSEC negotiation with router_c at that 
point, instead of as soon as isakmpd starts?

Thanks,

                -Matt-