FAQ : Connectivity (Tunnels and Subnets) : Heartbeat or AYIYA tunnel doesn't work

Heartbeat or AYIYA tunnel doesn't work

As stated on the Heartbeat and the AYIYA pages:

Warning Keep your machine NTP synced, if the timestamp difference is larger than 120 seconds the heartbeat/ayiya packets will be silently dropped. Note also that you need to select the correct time zone. Also keep attention that even though AICCU might accept the time when it runs, when your clock drifts (that is becomes unsynced) packets later during the day/week/month might not be accepted anymore.

Clocks which are out of sync too long will have the tunnel automatically and silently disabled. When the clock is out of sync a notification will be sent.

When your ISP doesn't have an NTP server you can use one from the NTP Server Pool project, see their pages for more information.

Other things to check:

Check the reachability of the PoP by tracerouting to the PoPs IPv4 address.
Check that, when you are behind a NAT, the forwarding in the NAT works. Configure it apropriatly, eg by setting your endpoint as the DMZ host or by forwarding proto-41 to that machine.
Investigate using tcpdump or Wireshark that packets are sent from your endpoint to the PoP and of course that you receive packets back.
Check for proto-41 or AYIYA activity.