Users Guide

862 Spanning Tree Protocol
transitioning of the port to Forwarding). The difference between RSTP and
the traditional STP (IEEE 802.1d) is the ability to recognize full-duplex
connectivity and ports which are connected to end stations, resulting in rapid
transitioning of the port to the Forwarding state and the suppression of
Topology Change Notifications.
MSTP is compatible with both RSTP and STP. It behaves appropriately when
connected to STP and RSTP bridges. A MSTP bridge can be configured to
behave entirely as a RSTP bridge or a STP bridge.
How Does STP Work?
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 priority, the switch with the lowest MAC address becomes 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. Both
internal and external path costs can be configured. For STP, RSTP, and the
MSTP CIST, only the external path costs are utilized in the lowest path cost
calculation. The internal path cost is used by the MST instances.
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.