I could be wrong, but I think the seed co. I bought my fennel seeds from said that most fennel is an annual, but it's the bronze fennel that's perennial. That's what I bought. Of course all varieties self seed heavily, so there's that. My original plant is a beautiful plant, growing five or six feet tall and almost as broad. That did change my original plans for my "herb bed".
Like cilantro, I end up with at least a gallon bag of seeds, which are not only good in cooking (a little goes a long way), but great for sharing.
I've also had luck with just stabbing a spade straight down into the base to get roots to share, much as one would do in dividing other perennials. Ironically, the volunteers that came from seed seem to sink their root deep before much top growth, so are harder to get out. If they are in an inconvenient spot, I just chop them off knowing I'll have to do it again. Heavy mulch helps.
I have not noticed alliopathic symptoms. They seem happy with cilantro, tomatoes, and hollyhocks ~ three other reliable self seeders. They are spreading though, so I will keep an eye on my spinach and lettuces.