spanning tree protocol
生成树（spanning tree），即用两个网桥来连接两个相同的计算机网络段，是一项协议，允许网桥进行信息的交流，这样只需要其中一个网桥来处理两台计算机之间发送的信息。生成树协议（spanning tree protocol，STP）可以防止网桥环路的发生。
The Spanning Tree Protocol (STP) is a network protocol that ensures a loop-free topology for any bridged Ethernet local area network. The basic function of STP is to prevent bridge loops and the broadcast radiation that results from them. Spanning tree also allows a network design to include spare (redundant) links to provide automatic backup paths if an active link fails, without the danger of bridge loops, or the need for manual enabling/disabling of these backup links.
Spanning Tree Protocol (STP) is standardized as IEEE 802.1D. As the name suggests, it creates a spanning tree within a mesh network of connected layer-2 bridges (typically Ethernet switches), and disables those links that are not part of the spanning tree, leaving a single active path between any two network nodes.
Mesh networking (topology) is a type of networking where each node must not only capture and disseminate its own data, but also serve as a relay for other nodes, that is, it must collaborate to propagate the data in the network.
暂时理解为，对于一个比较复杂的网络，有的节点作为中继，等等，创建一个生成树（不一定是最小的）保证没有环路和避免广播风暴
MacVTap
Macvtap is a new device driver meant to simplify virtualized bridged networking. It replaces the combination of the tun/tap and bridge drivers with a single module based on the macvlan device driver. A macvtap endpoint is a character device that largely follows the tun/tap ioctl interface and can be used directly by kvm/qemu and other hypervisors that support the tun/tap interface. The endpoint extends an existing network interface, the lower device, and has its own mac address on the same ethernet segment.Typically, this is used to make both the guest and the host show up directly on the switch that the host is connected to.
另一种简化的虚拟网桥设备驱动？
Since 0.9.4, QEMU and KVM only, requires Linux kernel 2.6.34 or newer
Libvirt默认创建了virbr0网桥，作为default虚拟网络的网关，并打开了STP。
STP 没有搜到是个什么意思
HMP or QMP是什么意思?
watchdog