Smith, 36, has committed himself to an eighth season at Franklin's Gardens after joining Northampton from French club Brive in 2001.
He said: "Staying at Northampton was an easy decision. It is a club where I have been for a long time, and a club for which I have a lot of affection."
Broster was capped twice on Wales' North America tour in 2005, and before joining Llanelli he featured for Saints' Premiership rivals Saracens.
Northampton rugby director Jim Mallinder added: "The front-row is an area of the squad where you cannot have too much depth.
"Tom's quality was apparent throughout last season. He had a fantastic campaign and he also has plenty of experience, which is another important commodity in the Premiership.
"Ben is an under-rated player who has proved his worth at the highest level. He is good in the tight, mobile around the park and also has international experience at all age groups."