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Technical basics

3.8 Spanning Tree

SCALANCE W770/W730 to IEEE 802.11n Web Based Management

Configuration Manual, 11/2014, C79000-G8976-C323-03

Root bridge and bridge priority

The identification of the most efficient connection is always related to the root bridge, a

network component that can be considered as a root element of a tree-like network

structure. With the "Bridge Priority" parameter, you can influence the selection of the root

bridge. The computer with the lowest value set for this parameter automatically becomes the

root bridge. If two computers have the same priority value, the computer with the lower MAC

address becomes the root bridge.

Response to changes in the network topology

If nodes are added to a network or drop out of the network, this may affect the optimum path

selection for data packets. To be able to respond to such changes, the root bridge sends

configuration messages (BPDUs) at regular intervals. You can set the interval between two

configuration messages with the "Hello Time" parameter.

Keeping configuration information up to date

With the "Max Age" parameter, you set the maximum age of configuration information. If a

bridge has information that is older than the time set in Max Age, it discards the message

and initiates recalculation of the paths.

New configuration data is not used immediately by a bridge but only after the period

specified in the "Forward Delay" parameter. This ensures that operation is started with the

new topology only after all the bridges have the required information.

3.8.1

RSTP, MSTP, CIST

Rapid Spanning Tree Protocol (RSTP)

One disadvantage of STP is that if there is a disruption or a device fails, the network needs

to reconfigure itself: The devices start to negotiate new paths only when the interruption

occurs. This can take up to 30 seconds. Fur this reason, STP was expanded to create the

"Rapid Spanning Tree Protocol" (RSTP, IEEE 802.1w). This differs from STP essentially in

that the devices are already collecting information about alternative routes during normal

operation and do not need to gather this information after a disruption has occurred. This

means that the reconfiguration time for an RSTP controlled network can be reduced to a few

seconds.

This is achieved by using the following functions:

● Edge ports (end node port)

Edge ports are ports connected to an end device.

A port that is defined as an edge port is activated immediately after connection

establishment. If a spanning tree BPDU is received at an edge port, the port loses its role

as edge port and it takes part in (R)STP again. If no further BPDU is received after a

certain time has elapsed (3 x hello time), the port returns to the edge port status.

● Point-to-point (direct communication between two neighboring devices)

By directly linking the devices, a status change (reconfiguration of the ports) can be made

without any delays.

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