Hi,
Based upon the description of the issue, I think your IPSEC Tunnel Configuration is Dynamic to Static. What I mean by that is, one end of the VPN Tunnel is configured to accept dynamic connections. In your case, NewYork is configured to Dynamic Connections and that is why London has to initiate the connection to bring up the tunnel. Once the tunnel is up, traffic will flow fine without any issue.
Please see the below example for details.
http://www.cisco.com/en/US/tech/tk583/tk372/technologies_configuration_example09186a008051a69a.shtml
So, in your case unless you configure Static to Static to bring up the tunnel from London or NewYork or constantly keep generating traffic from London to New York.
I hope it helps.
Regards,
Arul
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