When multiple switches are arranged in a simple hierarchical tree, switching loops are unlikely to occur. However, switched networks are often designed with redundant paths to provide for reliability and fault tolerance. While redundant paths are desirable, they can have undesirable side effects. Switching loops are one such side effect. Switching loops can occur by design or by accident, and they can lead to broadcast storms that will rapidly overwhelm a network. To counteract the possibility of loops, switches are provided with a standards-based protocol called the Spanning-Tree Protocol (STP). Each switch in a LAN using STP sends special messages called Bridge Protocol Data Units (BPDUs) out all its ports to let other switches know of its existence and to elect a root bridge for the network. The switches then use the Spanning-Tree Algorithm (STA) to resolve and shut down the redundant paths.

Each port on a switch using Spanning-Tree Protocol exists in one of the following five states:

• Blocking
• Listening
• Learning
• Forwarding
• Disabled

A port moves through these five states as follows:

• From initialization to blocking
• From blocking to listening or to disabled
• From listening to learning or to disabled
• From learning to forwarding or to disabled
• From forwarding to disabled

The result of resolving and eliminating loops using STP is to create a logical hierarchical tree with no loops. However, the alternate paths are still available should they be needed.

Interactive Media Activity Crossword Puzzle: Spanning Tree States After completing this activity, the student will be able to identify the function of spanning tree states.

   

Web Links Understanding Spanning-Tree Protocol http://www.cisco.com/univercd/cc/td/doc/ product/rtrmgmt/sw_ntman/cwsimain/ cwsi2/cwsiug2/vlan2/stpapp.htm