(GNS3 is a graphical interface simulator, which is much more convenient to build topology than Xiao Fan. At the same time, there are packettrace annotation and drawing tools, which brings great convenience in daily experiments and teaching. In terms of the size of the topology file, the space occupied by it is much smaller than that of Xiao Fan. Only a few K in size. Mainly to demonstrate the building function of GNS3 topology.)