POX启动时使用命令：openflow.discovery openflow.spanning_tree --no-flood --hold-down
来自：https://openflow.stanford.edu/display/ONL/POX+Wiki/
原文：
openflow.spanning_tree
This component uses the discovery component to build a view of the network topology, constructs a spanning tree, and then disables flooding on switch ports that aren't on the tree. The result is that topologies with loops no longer turn your network into useless hot packet soup.
Note that this does not have much of a relationship to Spanning Tree Protocol. They have similar purposes, but this is a rather different way of going about it.
The samples.spanning_tree component demonstrates this module by loading it and one of several forwarding components.
This component has two options which alter the startup behavior:
--no-flood disables flooding on all ports as soon as a switch connects; on some ports, it will be enabled later.
--hold-down prevents altering of flood control until a complete discovery cycle has completed (and thus, all links have had an opportunity to be discovered).
Thus, the safest (and probably the most sensible) invocation is openflow.spanning_tree --no-flood --hold-down .