Moxa EDS-82810G [43/113] The stp rstp concept

EDS-828 Series Featured Functions

3-30

The STP/RSTP Concept

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

from loops. Networks that have a complicated architecture are prone to broadcast storms caused by

unintended loops in the network. The EDS-828’s STP feature is disabled by default. To be completely effective,

you must enable RSTP/STP on every EDS-828 connected to your network.

Rapid Spanning Tree Protocol (RSTP) implements the Spanning Tree Algorithm and Protocol defined by IEEE

Std 802.1w-2001. 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 EDS-828. This feature is

particularly helpful when the EDS-828’s 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 RSTP and STP section in this chapter.

NOTE

The STP protocol is part of the IEEE Std 802.1D, 1998 Edition bridge specification. The explanation given

below 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 the most 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 them 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 because this path has a greater bandwidth and is therefore more efficient.