[pgrouting-users] assign vertex id only for limited area


I am searching for an option to use the assign_vertex_id function just for a special area. I dont want to recalculate the whole network, because it is very large and takes a long time. Is it possible to use the function only on a boundingbox or something? As i understand the function, it drops the whole vertices_tmp table at the beginning, so it looks like if my idea is hard to realize. If somebody has an idea, please let me know!
Thanks in Advance!