Comment 3 for bug 1691071

Issue is with tcp_tw_recycle set. This is not recommended in many scenarios. This is affecting time-stamps, which in turn is causing kernel to drop the SYN packets

    13432 SYNs to LISTEN sockets dropped

Disabling tcp_tw_recycle solved the issue. This needs to be added to provisioning. (or to reimage...)

Following command helped to resolve the issue vindicating above points.
sysctl -w net.ipv4.tcp_tw_recycle=0; service contrail-control restart

https://serverfault.com/questions/583488/no-response-to-some-syn-packets-when-timestamps-are-enabled