iRule persistance based on HTTP header

There are no "match across" option enabled on any of the persistence profiles I'm using. But I was using the same source address persistence profile on both virtual servers.

So I created a different source address persistence profile for my virtual server and set it as the fallback persistence profile. I still saw the same behavior as before.

However, the good news is that when I set the fallback persistence profile to None on my virtual server, then it started working as I expected it to. 2 of my 4 test requests went to one node and the other 2 went to the other.

I guess I don't understand why it is working this way. I thought the fallback persistence profile only took effect if my iRule did not match. Is there something I need to do in my iRule to prevent it from using the fallback persistence profile?

Thanks.

--Greg