Dell N2000 Series, Configuration Manual

Page: 716 / 1460

716
Configuring the Spanning Tree Protocol
transitioning of the port to Forwarding). The difference between the RSTP
and the traditional STP (IEEE 802.1d) is the ability to configure and
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 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. 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.