I have to extract two upper teeth due to an infection. is it necessary to do an antibiotics round before/after? mildly allergic to it .

Usually before: Usually dentists prefer that you have a round of antibiotics before they extract an infected tooth. If you do not then there is a chance of spread of the infection into the bloodstream.

Antibiotic Required: Antibiotics will change the pH of the localized tissue to one more helpful for profound anesthesia, as well as allow more favorable tissue for suturing. Not to mention, the potential spread of infection during the surgery. No surgery is 'routine', so proper care must always be given to allow the patient the best possible outcome potential. I recommend at least 5-days on the antibiotic pre-op.

Depends: Depends on symptoms. If teeth are "infected" but no pain, just extract teeth. If painful, antibiotics until surgery, then don't need them. If soft tissue infection, before and after. There are many variations depending on circumstances.