In 1821, the four Rodenbach brothers (Pedro, Alexander, Ferdinand and Constantijn) invested in a small brewery in Roeselare, in the West Flanders province of Belgium. The brothers agreed to a partnership for 15 years. At the end of this period, Pedro and his wife, Regina Wauters, bought the brewery from the others and Regina ran the business while Pedro served in the military. Their son Edward later took over the brewery (1864) and, it was during his directorship that the brewery saw great growth.
Edward's son, Eugene, took over in 1878 and, in preparation for this position, travelled to England where he learned how to ripen beer in oak barrels and then mix old and young beers. It was this that became the method of producing beer that Rodenbach became famous for. As Eugene produced no male offspring, a public limited liability corporation was created and most shares remained in the hands of descendants of the Rodenbachs until 1998 when the brewery was sold to Palm Brewery.
There are 4 different types of Rodenbach beers:
1. Rodenbach Original (5,2% ABV) blended from aged and young ale (25%/75%);
2. Rodenbach Grand Cru, a rich winey beer (6% ABV) which, although blended, contains less young ale (67%/33%) (in the past this beer was unblended aged beer from a single cask selected for its qualities by a tasting panel. This type of beer has now been renamed Rodenbach Vintage beer and is available in limited quantities);
3. Vin de Céréale, an aged beer that is 10% ABV.
4. In commemoration of the brewery 150th year of activity, it produced Rodenbach Alexander (5,2% ABV). It was a variant of the Original one with a cherry taste. It is name comes from the brewery founder Alexander Rodenbach.
After the take-over, Palm quickly stopped the production of Rodenbach's Alexander beer, a cherry-flavoured beer. When Palm Brewery bought Rodenbach, another brewery it owned already made a cherry beer. At that time Alexander Rodenbach had to be ceased for contractual non-compete sorts of reasons. However, in recent years, Palm/Rodenbach has produced and distributed, first, Rodenbach foederbier, which is served only from cask, and is unfiltered and unblended. It comes straight from an oak riping barrel and is not processed further.
I think the bartender told you to drink your Rodenbach with a shot of cherry liquor, so that you can taste the original Rodenbach Alexander (the cherry-flavoured beer, Rodenbach's erstwhile cherry beer which produced before the brewery was sold to Palm).