Moxa IKS-6728A-4GTXSFP-24-24-T [21/37] The stp rstp mstp concept

Managed Ethernet Switch Redundancy Protocol (UI 2.0) STP/RSTP/MSTP

5-2

The STP/RSTP/MSTP 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. Moxa switches’ STP feature is disabled by default.

To be completely effective, you must enable RSTP/STP on every Moxa 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 backwards 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 the

Differences between STP, RSTP, and MSTP section 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.”

What is STP?

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.

If STP is enabled, it will detect duplicate paths and prevent, or block, one of the paths from forwarding traffic.

In the following example, STP determined that traffic from LAN segment 2 to LAN segment 1 should flow

through bridges C and A since this path has a greater bandwidth and is therefore more efficient.

Bridge B

Bridge C

LAN 1

LAN 2

LAN 3

Bridge A