. Performing packet transfer-Bridges either filter, ignore, or forward packets. Using the Spanning Tree Protocol (STP)-Bridges and switches use STP to provide a loop-free topology for any bridged LAN. Loops occur when there are alternate routes between hosts, creating multiple active paths between stations. Without STP, it is possible that two connections might be simulta- neously live, which could result in an endless loop of traffic on the LAN. STP works by creating a tree that spans all the switches in an extended net- work and disables all alternate or redundant links that are not part of that tree. However, the disabled or blocked redundant links are available as a backup in case the initial link fails, supporting path redundancy. Bridge PDUs (BPDUs) are the frames that carry the STP information. LAN Interconnection and Internetworking Figure 6.11 illustrates a local bridge installed between two LAN segments located at the same local premises. When the bridge is plugged in, it sends out a hello message to its community and learns addresses by snooping on other traffic, and then the bridge builds an addressing table. Say that PC A wants to send a doc- ument to printer 1. The bridge realizes that the printer resides within its commu- nity. It knows the address and it therefore does not do anything except filter the packet. On the other hand, if PC A is attempting to communicate with server Z, the bridge says, "I don't know where that server is. It's not part of my local commu- nity, SO it must be somewhere else on the other side of this bridge." The bridge then broadcasts that information to the other side of the bridge. In essence, the bridge creates broadcast storms. Bridges are not networkable devices; that is, they can't target a destination net- work. They can only determine whether a destination is or is not on its segment, and if the destination is somewhere else, the bridge sends a message to every some- where else that it knows about. This can be an especially big problem if you use a bridge in a remo