The standout team in Pro14 could only manage a 3-0 win against winless Zebre as Leinster struggled on Saturday.
Leinster made it four wins from four in Pro14 as they consigned Zebre to a fourth straight defeat, but they had to settle for a 3-0 margin of victory on Saturday.
Top defeated bottom in Conference A, but there was surprisingly little between the two sides as Zebre earned a losing bonus point in testing conditions in Parma.
The Italian side were much improved from their previous losses, conceding at least 50 points in each of them, yet had no answer to Ross Byrne's 20th-minute penalty.
Leinster are now the only team with a 100 per cent record, after Scarlets' winning start was ended in comprehensive fashion by Edinburgh.
Duhan van der Merwe scored a hat-trick of tries and Mike Willemse grabbed another two as the Scottish outfit crushed Scarlets 46-7.
Connacht sit top of Conference B after edging out Cheetahs 24-22 in a dramatic finish.
Cheetahs centre Dries Swanepoel was sent off with 10 minutes remaining and his side 22-14 up, watching on as Tom Farrell subsequently crossed twice to steal victory.
Zebre are not alone with four defeats from four, as Southern Kings were beaten again, going down 36-30 to Benetton - the victors breaking their own duck.
In the only other match of the day, Dragons eased past Glasgow Warriors, who did not get on the scoresheet until the final minute of an 18-5 reverse.