Quanta Cloud Technology QuantaMesh QNOS5, Configuration Manual

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The switches (bridges) that participate in the spanning tree elect a switch to be the root bridge for the
spanning tree. The root bridge is the switch with the lowest bridge ID, which is computed from the unique
identifier of the bridge and its configurable priority number. When two switches have an equal bridge ID
value, the switch with the lowest MAC address is the root bridge.
After the root bridge is elected, each switch finds the lowest-cost path to the root bridge. The port that
connects the switch to the lowest-cost path is the root port on the switch. The switches in the spanning
tree also determine which ports have the lowest-path cost for each segment. These ports are the
designated ports. Only the root ports and designated ports are placed in a forwarding state to send and
receive traffic. All other ports are put into a blocked state to prevent redundant paths that might cause
loops.
To determine the root path costs and maintain topology information, switches that participate in the
spanning tree use Bridge Protocol Data Units (BPDUs) to exchange information.
3.8.3. MSTP in the Network
In the following diagram of a small 802.1D bridged network, STP is necessary to create an environment with
full connectivity and without loops.
Figure 3-11: STP in a Small Bridged Network
Assume that Switch A is elected to be the Root Bridge, and Port 1 on Switch B and Switch C are calculated to
be the root ports for those bridges, Port 2 on Switch B and Switch C would be placed into the Blocking state.
This creates a loop-free topology. End stations in VLAN 10 can talk to other devices in VLAN 10, and end
stations in VLAN 20 have a single path to communicate with other VLAN 20 devices.
Figure 12 shows the logical single STP network topology.