Forum Discussion
With a fastL4 vip hardware acceleration should be in use if your platform supports it. If that is the case the traffic would not be visible to tcpdump after the SYN to the servers (I can't remember if the SYN itself would be visible between the LTM and the servers if ePVA is being used). So it's possible that the some traffic is leaving the LTM and you just can't see it without disabling hardware acceleration for that vip.
That said, there are many things that could cause what you describe and we don't have enough information to draw any conclusions. Would you be able to post your virtual-server, pool, and self-ip info? Are the servers on the same network as the LTM, or is the LTM using a GW to reach them? Is the functional VIP on the same LTM, or do you mean that a similar vip on a different LTM works fine? Does the LTM successfully handle traffic for any other servers on the same network as these servers, or are these the only servers on that network that the LTM supports?