The Liberty Stadium win puts the Dubliners in an advantageous position in Pool 1 but they will be determined to continue their impressive start against the Top 14 champions.
Brian O'Driscoll remains a doubt for the clash but Leinster have been boosted by the arrival of South African full-back Zane Kirchner following his involvement in the Rugby Championship.
The two sides met at the pool stages in 2008 - honours were even but Leinster ran-out emphatic 33-3 winners in Dublin.
Castres also triumphed in Round One with a 19-13 victory over Northampton Saints, thanks to Romain Martial's interception try.
The French club have not reached the knockout stages of the Heineken Cup since losing a semi-final to Munster in 2002. And they will be without key trio Piula Faasalele, Yannick Forestier and Marcel Garvey due to injury.
Leinster's defeat to Clermont last season ended a series of 12 consecutive victories over French clubs at home, a run that included a 33-3 win over Castres in 2008.
Castres have won just two of their last 21 away games in the Heineken Cup (L18 D1) and in Ireland, they have lost all seven of their games.
Only Racing Métro 92 (161) made more tackles than Leinster (153) on the opening weekend of action in the Heineken Cup.
Fergus McFadden (14) and Gordon D'Arcy (13) made the most tackles by Leinster players in Round 1 with both maintaining a 100% success rate.