code: def alloc_vpcid(nexus_ips):
      
  
      LOG.debug("alloc_vpc() called")
  
      vpc_id = 0
      intersect = _get_free_vpcids_on_switches(nexus_ips)
      for intersect_tuple in intersect:
          try:
              update_vpc_entry(nexus_ips, intersect_tuple.vpc_id,
                               False, True)
              vpc_id = intersect_tuple.vpc_id
              break
          except Exception:
              LOG.exception(
                  "This exception is expected if another controller "
                  "beat us to vpcid %(vpcid)s for nexus %(ip)s",
                  {'vpcid': intersect_tuple.vpc_id,
                   'ip': ', '.join(map(str, nexus_ips))})
  
      return vpc_id
docs: Allocate a vpc id for the given list of switch_ips.