Background: Know: Recognize: 




Spanning Tree ProtocolEdit

Spanning Tree Protocol is a link management protocol that provides path redundancy while preventing undesirable loops in the network. For an Ethernet network to function properly, only one active path can exist between two stations. You can see more details here.[spanning tree protocol]


STP switch port states:Edit

  1. Blocking -(20s) A port that would cause a switching loop, no user data is sent or received but it may go into forwarding mode if the other links in use were to fail and the spanning tree algorithm determines the port may transition to the forwarding state. BPDU data is still received in blocking state. Prevents the use of looped paths.
  2. Listening - (15s)The switch processes BPDUs and awaits possible new information that would cause it to return to the blocking state. It does not populate the MAC address table and it does not forward frames.
  3. Learning - (15s)While the port does not yet forward frames it does learn source addresses from frames received and adds them to the filtering database (switching database). It populates the MAC Address table, but does not forward frames.
  4. Forwarding - A port receiving and sending data, normal operation. STP still monitors incoming BPDUs that would indicate it should return to the blocking state to prevent a loop.
  5. Disabled - Not strictly part of STP, a network administrator can manually disable a portHelps NM

Helps NM

Physical topology can contain loops(e.g. poor documentation and deliberate redundancy for fault tolerance), also it will automatically respond to link/node failures.

Some redundant links are added into the network in order to improve the reliability of network. As a result, some loops will be created in the physical topologies. A goal of redundant topologies is to eliminate network outages caused by a single point of failure. Althoughredundant links are useful, they also bring some serious problems, such as the broadcast storm, multiple frame copies and MAC address table instability. Data frames can be simultaneously broadcasted to all redundant links. As a result, they will cause network loop. The Spanning Tree Protocol was originally designed for bridges to address the problem of network loop. It is also applied to LAN switches and routers operating as a bridge. STP ensures all bridged segments are reachable but any points where loops occurred will be blocked.


another description originally from slide AU

The Spanning Tree Protocol (STP) is a network protocol that ensures a loop-free topology for any bridged Ethernet local area network. The basic function of STP is to prevent bridge loops and ensuing broadcast radiation. Spanning tree also allows a network design to include spare (redundant) links to provide automatic backup paths if an active link fails, without the danger of bridge loops, or the need for manual enabling/disabling of these backup links.