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What is STP?
STP represents Spanning Tree Protocol. It is a link management protocol created to provide a redundant connection that pause switching loops in the STP web. It is a Layer 2 protocol that runs on bridges and switches, which must be allowed on the switch interfaces.
In an STP environment, the switches exchange data between themselves using bridge protocol data units (BPDU) and will then accept in on all ports for this BPDU message. Because a bridge is turned on, it necessarily considers that it is the root bridge in the STP tree.
The STP application selects a root bridge and evaluates all paths from the lower bridges back to it. It is provided that hardware failure of a root bridge in the redundant environment, a new root is chosen and port paths can be recalculated.
The selection procedure of the Root Bridge occurs each time when a network change appears like a new switch is inserted in the network topology, or an current switch is deleted or the modern Root Bridge is failed.
If multiple switches of the network do not get BPDUs from the Root Bridge within 20 seconds, they consider that the Root Bridge has failed. If the modern Root Bridge fails, remaining switches necessarily begin the election procedure to select a new Root Bridge again.
It depends on the connected media link. STP creates a value to every port of the network. This value is referred to as the port cost value. STP uses this value to select the individual best path when several links are available among two switches. It can choose the port which has the minimum port cost value.
Path cost is an acquired value of the port costs from the Root Bridge to multiple switches in the network. It is always evaluated from the Root Bridge. Default path value at the Root Bridge is 0. BPDU includes the path cost data.
When the Root Bridge communicates BPDU out from its link, it sets the path value to 0. The switch which gets these BPDU increments the path value by inserting the port cost value of the port on which the BPDU appeared.
A designated port is a port that has the minimum port cost value to get on a given network, distinguished from several ports on that segment. STP indicates the designated ports as the promoting ports.
A non-designated port is a port that has a larger port cost than the designated port. STP indicates the non-designated port as the blocking port and it is used to eliminate loops.
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