[gentoo-user] GRE link state detection

[thegeezer <thegeezer@thegeezer.net>]

Howdy all,
i was wondering if anyone has any idea if there is a means by which i
can detect GRE link state ?

what i have is two sites each with two very unstable internet links
in order to vpn between them i have ipsec tunnels linking each side
twice (four ipsec tunnels in total)
i then have 4x GRE tunnels over the top of those in order that i have a
secured routable VPN
this gives me net.vpn0 net.vpn1 net.vpn2 and net.vpn3
finally i run BIRD over the top which works very well, and synchronises
routing tables between the two sites, and allows for me to do such fun as
# /etc/init.d/net.vpn0 stop
and watch all traffic automagically cut over to another link.

so far so awesome.

however, as i said the internet links are very unstable, and sometimes
just blackhole. so what i was hoping to do is just enable keepalives on
the gre tunnel.  which sadly seems to be cisco only. 
can anyone suggest a way of detecting if the GRE is not fully connected ?  
BIRD only fails over if the net.vpn0 device is down (ifconfig up/down)
and for the life of me i cannot find how to detect if a GRE tunnel is
'connected', it seems to just blindly send packets to the remote IP.
is my only choice to use L2TP instead ?

thanks in advance for any suggestions; if anyone would like the configs
in order to set this up let me know and i'll reply to list with a mini-howto
PS if i'm on the wrong list i'd welcome a friendly push to the correct list