填空题 1.  A ______ cloud is a collection of routers that are configured with either an mGRE interface or a point-to-point GRE interface (or a combination of the two) and that share the same subnet. ______网云是一个配置了mGRE接口或点到点GRE接口(或两者共存)的路由器集合，这些路由器共享相同的子网。