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Ethernet 31
Figure 2.24: Possible tree topology by means of (R)STP
Figure 2.24 shows a network with five different structure elements. Different redundant connections
are created. This results in the occurrence of unacceptable loops that will quickly
congest the network. The RSTP protocol converts this topology into a tree structure by closing
off a number of ports. One structure element is configured here as root. From this root,
all other switches can be reached via one single path. If a network error occurs, then a new
active path is created.
Extensions on RSTP
In order to meet the needs of the automation, many companies plan proprietary extensions
on the RSTP protocol in order to attain recovery times of less than a second. This way, QoS
is obtained for the redundant building of automation networks.
Fast Ring Detection is an extension of Phoenix Contact on the RSTP. When a network switch
drops out, recovery times of 100... 500 ms are reached. Recovery times of not more than
500 ms are available for extensive automation networks with 1000 entered address tables
in the switches. These times are shorter in case of less number of terminals in the network.
This protocol, however, can only be used for 10 or 200Mbps.
2.8.4 Bridge Protocol Data Units (BPDUs)
The tree structure is calculated by means of a specific algorithm so that there is one switch
configured as the root. Every switch must in fact have all the information required so as to
be able to define the correct port lines. In order to ensure that every switch has sufficient,
correct information, switches exchange information between them. Special frames are used
for this, Bridge Protocol Data Units (BPDUs).
A bridge sends a BPDU, with it as the SA using the unique MAC of the port itself and, as the
DA, the STP Multicast address 01:80:C2:00:00:00. There are various types of BPDU:
• Configuration BPDU (CBPDU) used for the calculation of the spanning tree
• Topology Change Notification BPDU (TCN), used to notify changes in the network
• Topology Change Notification Acknowledgement (TCA)