User Documentation

User Manual Managed Switches

IE-SW-VL05M/08M Series

IE-SW-PL06M/08M/09M/16M

Series

The second port from the last port

number

The last port of the switch (highest

port number)

IE-SW-PL18M Series

Port G1

Port G2

IE-SW-PL10M Series

Port G2

Port G3

3.5.6 STP / RSTP

3.5.6.1 The STP / RSTP Concept

Spanning Tree Protocol (STP) was designed to help reduce link failures on a network, and provide

an automatic means of avoiding loops. This is particularly important for networks that have a

complicated architecture, since unintended loops in the network can cause broadcast storms.

Weidmüller switches’ STP feature is disabled by default. To be completely effective, you must enable

RSTP/STP on every Weidmüller switch connected to your network.

Rapid Spanning Tree Protocol (RSTP) implements the Spanning Tree Algorithm and Protocol

defined by IEEE 802.1D-2004. RSTP provides the following benefits:

 The topology of a bridged network will be determined much more quickly compared to STP.

 RSTP is backward compatible with STP, making it relatively easy to deploy.

For example:

 Defaults to sending 802.1D style BPDUs if packets with this format are received.

 STP (802.1D) and RSTP (802.1w) can operate on different ports of the same switch, which is

particularly helpful when switch ports connect to older equipment such as legacy switches.

You get essentially the same functionality with RSTP and STP. To see how the two systems differ,

see section ‘Differences between STP and RSTP’ later in this chapter.

NOTE: The STP protocol is part of the IEEE Std 802.1D, 2004 Edition bridge specification. The

following explanation uses “bridge” instead of “switch.”

STP (802.1D) is a bridge-based system that is used to implement parallel paths for network traffic.

STP uses a loop-detection process to:

 Locate and then disable less efficient paths (i.e., paths that have a lower bandwidth).

 Enable one of the less efficient paths if a more efficient path fails.

The figure below shows a network made up of three LANs separated by three bridges. Each segment

uses at most two paths to communicate with the other segments. Since this configuration can give

rise to loops, the network will overload if STP is NOT enabled.