| A Layer 2 switch, which functions as a | | | | before they are opened for use, and redundant |
| transparent bridge, offers no additional links for | | | | links are shut down to prevent the loops from |
| redundancy purposes. To add redundancy, a | | | | forming. STP is communicated between all |
| second switch must be added. Now two switches | | | | connected switches on a network. Each switch |
| offer the transparent bridging function in parallel. | | | | executes the Spanning-Tree Algorithm |
| LAN designs with redundant links introduce the | | | | (STA)based on information received from other |
| possibility that frames might loop around the | | | | neighboring switches. The algorithm chooses a |
| network forever. These looping frames would | | | | reference point in the network and calculates all |
| cause network performance problems. | | | | the redundant paths to that reference point. |
| For example, when the switches receive an | | | | When redundant paths are found, STA picks one |
| unknown unicast, both will flood the frame out all | | | | path to forward frames with and disables or |
| their available ports, including the ports that link to | | | | blocks forwarding on the other redundant paths. |
| the other switch, resulting in what is known as a | | | | STP computes a tree structure that spans all |
| bridging loop, as the frame is forwarded around | | | | switches in a subnet or network. Redundant paths |
| and around between two switches. This occurs | | | | are placed in a blocking or standby state to |
| because parallel switches are unaware of each | | | | prevent frame forwarding. The switched network |
| other. The Spanning-Tree Protocol (STP), which | | | | is then in a loop-free condition. However, if a |
| allows the redundant LAN links to be used while | | | | forwarding port fails or becomes disconnected, |
| preventing frames from looping around the LAN | | | | the STA will run again to recomputed the |
| indefinitely through those redundant links, was | | | | Spanning-Tree topology so that blocked links can |
| developed to overcome the possibility of bridging | | | | be reactivated. By default, STP is enabled on all |
| loops. It enables switches to become aware of | | | | ports of a switch. STP should remain enabled in a |
| each otherso that they can negotiate a loop-free | | | | network to prevent bridging loops from forming. |
| path through the network. Loops are discovered | | | | |