Basic Spanning Tree Protocol (STP) Information
STP is used by switches to prevent loops occurring on a network, this process is implemented by using spanning tree algorithm in disabling unwanted links and blocking ports that could cause loop.
Loops and duplicate frames can have severe consequences on a network. Most LANs are designed to provide redundancy so that if a particular link fails another one can take over the forwarding of frame across the LAN.
Basically, each switch port on a network detects the MAC address of a host or PC A, it then sends messages to other switches on the network to inform them of it’s knowledge on how to get to PC A. The problem starts when another switch discovers the same host or PC A’s MAC address, In time every switch on the network will start flooding messages on the network of their discovery and how to get to the same PC A and a loop has formed.
STP ensures that there is only one logical path between all destinations on the network by intentionally blocking redundant paths that could cause a loop.
When a switch port detects a loop in the network, it blocks (A port is considered blocked when network traffic is prevented from entering or leaving that port) one or more redundant paths to prevent a loop forming.
To stop a loop from forming, STP chooses one switch to be ‘Root Bridge’ on the network. Then other switches selects one of its ports as ‘Root Port’ then, a ‘designated port’ is chosen on each segment and all other ports are closed down.
STP outline of Process
Cisco switches runs STP by default, no configuration needed.
STP continually monitors the network for failures, be it switchports or changes in the network topology. STP acts quickly in making redundant ports available if there is a failure on a link.
Spanning Tree Protocol
*Used by switches to turn a redundant topology into a spanning tree.
*Disables unwanted links by blocking ports
*Is defined by IEEE 802.1d
*Switches run STP by default - configuration needed.
*Choose one switch to be Root Bridge
*Choose a Root Port on each other switch
*Choose a Designated Port on each segment
*Intentionally closes down all other ports
More STP Info: