Hello Ashvin,
Thanks for your fast answer.
I checked the jsrpd files logs and it was very usefull.
In fact the swiches direclty connected to the firewall (nexus and 6880X) were configured with STP portfast/edge instead of porstfast trunk/edge trunk ...... which explain the 15s due to rstp (blocking to learning/forwarding state).
Now everything is Ok
Thanks,
Best regards,