Simple iRule not working - HTTP::respond KO

FastL4 by definition means Layer-4 only. It is faster because the system only processes the data up to Layer4, which is accelerated using the ePVA hardware where available.

The GUI will not allow you to attach a Layer 7 profile to such a VIP, because it will be trying to access Layer 7 data on a pipeline that is only aware of Layer 4.

However, to enable the use-case of PEM (Policy Enforcement Manager) which needs to inspect higher layer traffic and still allow such flows to be handled at maximum velocity, you are allowed to attach a HTTP profile on fastl4 vips as documented in the solution article I linked. PEM is typically used in Service Provider environments where the volumes are quite high and where they benefit from such acceleration.

Since the inspection in PEM is read-only, this is a good tradeoff between capacity and capability. As soon as you want to do read-write, you have to evict the pipeline from hardware and into the CPU, and this requires the VIP to be of Standard Type.

What you are trying to do is simply not possible, and the GUI wouldn't even allow you to do it if it weren't for the fact that PEM needs it.

If you want to insert XFF, then use standard VIP and configure your http profile appropriately. If you want maximum performance, then avoid the use of irules entirely, and use Local Traffic Policies instead wherever possible.