Every host of a news server maintains agreements with other nearby news servers to synchronize regularly. In this way news servers form a redundant network. When a user posts to one news server, the post is stored locally. That server then shares posts with the servers that are connected to it for those newsgroups they both carry. Those servers do likewise, propagating the posts through the network. For newsgroups that are not widely carried, sometimes a carrier group is used for crossposting to aid distribution. This is typically only useful for groups that have been removed or newer "alt.*" groups. Crossposts between hierarchies, outside of the Big 8 and "alt.*" hierarchies, are prone to failure.