Eidolon HD advancment?

[Miranda Silva]

I confused of how Eidolon HD works.

Is it just like a character with class levels? Full HD at level one, roll for HP after each level gain.

OR

Is it more like monsters with HD were they get half the HD each level.

[Mojorat]

only players get full at first. as for advancement that seems up I'n th air and is likely up to your dm.