One Root Bridge
A tree topology must have a root. Therefore, the root bridge is introduced by STP.
There is only one root bridge on the entire STP-capable network. The root bridge is the logical center of but is not necessarily at the physical center of the entire network. The root bridge changes dynamically with the network topology.
After the network converges, the root bridge generates and sends out configuration BPDUs at specific intervals. The other devices forward only the configuration BPDUs to advertise the changes in the topology to ensure a stable network.
The spanning tree is calculated based on two types of measurements: ID and path cost.
ID
IDs are classified into Bridge IDs (BIDs) and Port IDs (PIDs).
BID
IEEE 802.1D defines that a BID is composed of a 16-bit bridge priority and a bridge MAC address. The bridge priority occupies the leftmost 16 bits and the MAC address occupies the rightmost 48 bits.
On an STP-capable network, the device with the smallest BID is selected to be the root bridge.
PID
The PID is composed of a 4-bit port priority and a 12-bit port number. The port priority occupies the left most 4 bits and the port number occupies remaining bits on the right.
The PID is used to select the designated port.
The port priority affects the role of a port in a specified spanning tree instance.
Path cost
The path cost is a port variable and is used to select a link. STP calculates the path cost to select a robust link and blocks redundant links to trim the network into a loop-free tree topology.
On an STP-capable network, the accumulative cost of the path from a certain port to the root bridge is the sum of the costs of all the segment paths into which the path is separated by the ports on the transit bridges. The rate of an aggregated link is the sum of the rates of all Up member links in the aggregated group.
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