Spanning Tree

Spanning Tree:
1)Spanning trees are a standard technique used in local area connection. Spanning tree algorithms were developed to prevent redundant transmission of data along intermediate hops between a source and destination host on a mesh network topology. Without spanning trees, a mesh network can be flooded and rendered unusable by messages circulating in an infinite loop between hosts.
2)An algorithm used in transparent bridges that dynamically determines the best path from source to destination. It avoids bridge loops (two or more paths linking one segment to another), which can cause the bridges to misinterpret results.
The algorithm creates a hierarchical "tree" that "spans" the entire network including all switches. It determines all redundant paths and makes only one of them active at any given time. The spanning tree protocol (STP) is part of the IEEE 802.1 standard.

How STP Works:
When STP initially comes online in a network, one of its first actions is to use the STA(Spanning Tree Algorithm) to select a root bridge and a root port. The root bridge is the bridge with the lowest-value bridge identifier. Switches or bridges using STP exchange multicast frames called Bridge Protocol Data Units. All the switches on the network use these BPDUs to broadcast their bridge IDs to the other switches in the network. After the root bridge is selected, the root ports on all other bridges are determined.

In the figure below, Switch A is acting as the root bridge, calculating the least-cost path to switch D. Notice the numbers associated with the root bridge's path to each invididual destination; the path with the lowest number hs the highest priority. The higher the number between invidiual segments, the higher the coast of transmitting a frame between those two segments. The port through which the root bridge can be reached with the least amount of hops or cost determines a bridge's root port; this is referred to as the least path cost.
The lowest calculated path is not always the most ideal path. For example, if mutliple high-speed links to a destination exist, the links may total more than the cost of a very slow link, such as a modem. Even though the straight path has the fewest hops, it is much slower than using a high-spped, longer path. To over this problem, you as the administrator can manually change a slower-speed link to have a higher port cost, which STP will use to calculate a higher path cost. The goal is to make changes to the network so that the fastest, most efficient route to the root port is designated for the switch to user. The fastest links should always have the lower port costs.
A designated bridge is the bridge, or switch on each LAN that provides the shortest route with the least path cost. The designated bridge is the onley bridge that is allowed to forward frames to and from the other bridges.A designated prot on the switch is the port that connects the switch to the physical interface of the designated bridge.

Types of frames:(These are the frames that will cause strom)
Broadcast frames
Multicast frames
Unknown unicasts

Various States of STP

1)Blocking
2)Listening
3)Learning
4)Forwarding
5)Disabled.

Thanks
Karthick M