Abstract: The results of analyzing BGP routing tables show that there are a large number of routes of which the mask length of prefix exceeds the maximum mask length as specified by the address allocation policies. Based on the mask length of prefix, a method is presented to slow the growth of BGP routing table down so as to filter routes. For the prefixes of which the mask lengths are longer than those specified by the address allocation policies, they are only allowed to be announced in a certain range of distance. Once the route is traversing the range of given number of autonomous systems, it will be filtered out without announcement. In order to implement the filtering mechanism, the BGP is modified to add a new route attribute in it so as to record the maximum announced distance of the filtered route, thus implementing the route filtration. The method is proved feasible through simulation by using actual BGP routing tables and efficient for restraining the growth of BGP routing tables.