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RS400 145 ROS™ v3.5
Design Considerations for RSTP in Ring Backbone Configurations
1. Select the design parameters for the network.
What are the requirements for robustness and network failover/recovery times? Typically, ring
backbones are chosen to provide cost effective but robust network designs.
2. Identify required legacy support and ports with half duplex/shared media restrictions.
These bridges should not be used if network failover/recovery times are to be minimized.
3. Identify edge ports
Ports that connect to host computers, IEDs and controllers may be set to edge ports in order to
guarantee rapid transitioning to forwarding as well as reduce the number of topology change
notifications in the network.
4. Choose the root bridge.
The root bridge can be selected to equalize either the number of bridges, number of stations or
amount of traffic on either of its legs. It is important to realize that the ring will always be broken
in one spot and that traffic always flows through the root
5. Assign bridge priorities to the ring.
The strategy that should be used is to assign each bridge’s priority to correspond to its distance
from the root bridge. If the root bridge is assigned the lowest priority of 0, the bridges on either
side should use a priority of 4096 and the next bridges 8192 and so on. As there are 16 levels of
bridge priority available, this method provides for up to 31 bridges in the ring.
6. Implement the network and test under load.
5.3.3 RSTP Port Redundancy
Figure 98: Port Redundancy
In cases where port redundancy is essential, RSTP allows more than one bridge port to service
a LAN. For example, if port 3 is designated to carry the network traffic of LAN A, port 4 will
block. Should an interface failure occur on port 3, port 4 would assume control of the LAN.