Is there a way for the LTM to insert X-Forwarded-For Header for VIP configured with fastL4 Profile? It's a performance layer 4 VIP. On a standard VIP, there is a HTTP profile where we can customize and insert XFF, but not on fast L4. Any ideas?
No, it's not possible, that's the point of the FastL4 profile - cutting out all that stuff. Accept the connection, load balance it and then cut it to the ASIC, it's not very fast if you're still scanning it for HTTP requests etc.