STP Root Bridge, Root Port and Designated Port Election Principles
Election principles for root bridges, root ports and designated ports
The Root Identifier, Root Path Cost, Bridge Identifier and Port Identifier fields carried in the BPDU packet constitute a message priority vector {Root Bridge ID, Root Path Cost, Sending Device BID, Sending Port PID}. Devices interact and compare the values of fields in the message priority vector to determine the root bridge, root port, and designated port.
- BID consists of bridge priority and bridge MAC address, with the upper 16 bits being bridge priority and the remaining lower 48 bits being MAC address.
- Root Path Cost RPC (Root Path Cost) is the cumulative value of the path cost of each port on each bridge that a port passes through to the root bridge. Path Cost is a port variable, which is the reference value used by Spanning Tree Protocol to select links. Spanning tree protocol prunes the network into a loop-free tree structure by calculating path cost, selecting stronger links and blocking redundant links.
- PID consists of two parts, the upper 4 bits are port priority, and the lower 12 bits are port number.
root bridge election
The principle of minimum BID: The smallest BID device is elected as the root bridge.
root port election
- The principle of least RPC: On non-root bridge devices, the port with the least RPC root path cost is elected root port.
- Principle of minimum sending device BID: If two or more ports on a non-root bridge device have the same root path cost, the port with the minimum sending device BID in the received BPDU message is elected as the root port.
The root path cost from each port on the root bridge to the root bridge is 0, and there is no root port on the root bridge.
Designated port election
Minimum PID principle: When the root path cost is the same, the port with higher PID value is blocked, and the port with lower PID value is elected as the designated port.
As shown in Figure 5-8, PID only works when the PID of port PA1 on DeviceA is smaller than that of port PA2. Since the root path overhead and sending device BID are the same in the BPDUs received on the two ports, PID is the only basis for eliminating loops.
Figure 5.8 Topology for comparison using PID
Updated on November 27, 2024
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