Al returned to his roots in joining the Warriors, from Edinburgh Rugby.
Al said: "I'm really enjoying my rugby with Glasgow. It looks like we're going places because we have a good squad of players."
He said: "I'm originally a Glasgow guy, so I'm glad to have signed for a longer period of time and to be part of this very optimistic club."
Sean Lineen added: "There's no doubt he will play a major part with the Scotland national team in the future and it's exciting he's shown commitment to stay with Warriors because he's well respected and is developing into a world-class performer."
A former pupil of Bishopbriggs High School, Al was part of the Glasgow Thistles squad that toured New Zealand in 2000.
Al has represented Scotland at under-19 and under-21 levels and remains attached to the Scottish Institute of Sport.
He played his club rugby initially with Allan Glen's before moving to Stirling County. His talent was noticed at Bridgehaugh and, with help from the Scottish Institute of Sport.
Al swiftly progressed through national age-group squads to the Edinburgh side, where he replaced Nathan Hines as a regular alongside Scott Murray.
Al was given his first full international chance by Matt Williams, one of a clutch of promising youngsters taken on tour to Australia and New Zealand in 2004.
His senior Scotland debut was in the non-cap tour game against Queensland Reds on the 2004 summer tour of Australia.
He followed that by coming on as a replacement in the games against New South Wales Country and New South Wales Waratahs.
Al went on to gain his first cap against Australia at Murrayfield in the opening match of the 2004 Abbey Autumn Tests, coming on as a replacement.
He made such a favourable impression on his arrival in the Glasgow squad that he was appointed captain.
Al made his first competitive appearance as captain against Newport Gwent Dragons at Hughenden on the Magners League's opening day.