only enable radv if upstream connection available

[Uwe Kleine-König]

Hello,

I'm a bit annoyed because my ipv6 tunnel (via sixxs) is unreliable from
time to time and I wonder if bird can help me to at least stop sending
out radv packets to my local machines if the tunnel is not working[1]. (Or
maybe send packets, but with a zero lifetime.)

I read about filters now and also spend some time googling, but still I
don't see if/how this could be possible.

Does someone of you has a nice idea or a link to an example that could
help me?

Best regards
Uwe

[1] for some definition of "not working"