DELL N3000 User Manual

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. 

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.