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STP Basic Concepts

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Root Bridge
A tree topology must have a root. As defined in STP, the device that functions as the root of a tree network is called the root bridge.

There is only one root bridge on the entire STP network. The root bridge is the logical center of but is not necessarily at the physical center of the network. The root bridge changes dynamically with the network topology.

After network convergence completes, the root bridge generates and sends configuration BPDUs to other devices at specific intervals. Other devices process and forward the configuration BPDU to communicate the topology changes to downstream devices.

Metrics for Spanning Tree Calculation
A spanning tree is calculated based on the following metrics: bridge ID (BID), port ID (PID), and path cost.


According to IEEE 802.1D, a BID is composed of a bridge priority (leftmost 16 bits) and a bridge MAC address (rightmost 48 bits).

On an STP network, the device with the smallest BID is elected as the root bridge.

IDs are classified into bridge ID (BID) and port ID (PID).

A PID is composed of a port priority (leftmost 4 bits) and a port number (rightmost 12 bits).

The PID is used to select the designated port.
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