Premiership champions Harlequins will face Castres and Cardiff Blues in the pool stages of next season's Heineken Champions Cup.
Exeter, the 2020 European Cup winners, were also drawn in tier one, and will face Montpellier and Glasgow Warriors.
The pool stages will start in December, with teams facing each other home and away - and so each playing four games.
The 24 competing teams were drawn into two pools of 12 - which were each then split into four tiers of three clubs.
In each pool, tier-one sides face tier-four teams, and clubs in tier two playing those in tier three.
Each pool's tier has three clubs - one each from the Premiership, Pro14 and Top 14 - with the tiers determined by domestic standings.
In the pool stage, teams do not face opponents from the same domestic league - so, for instance, a tier-one Premiership club will only play the Pro14 and Top 14 teams in tier four.
Defending champions Toulouse, who are in tier one, will face tier-four clubs Wasps and Cardiff Blues.
The 2021-22 Champions Cup final is scheduled to be in Marseille on 28 May next year.
Tier one: La Rochelle, Exeter Chiefs, Leinster
Tier two: Racing 92, Sale Sharks, Ulster
Tier three: Clermont Auvergne, Northampton Saints, Ospreys
Tier four: Montpellier, Bath, Glasgow Warriors
Tier one: Toulouse, Harlequins, Munster
Tier two: Bordeaux-Begles, Bristol Bears, Connacht
Tier three: Stade Francais, Leicester Tigers, Scarlets
Tier four: Castres, Wasps, Cardiff Blues