GE T1000, Technical Manual

Page: 60 / 302

Chapter 4
– Functions
GE Reason Switches
60 REASON SWITCHES-TM-EN-3
Figure 16: Ring topology LAN and possible paths for data traffic from IED A to IED B
The protocol works to create a logical topology that resembles a tree. The topology
will have a root; branches that extend from it until reaching the leaves. The first step
is to define where is the root of the tree, this means, who will be the root switch. The
root switch is the logical centre of the topology.
After bridges are defined, the ports that are part of this bridge must be defined. They
can be root port and designated port. A root port is the port of a designated bridge
that leads to the root switch, and designated ports forwards traffic away from the
root. If a port is not a designated port or a root port, it will be disabled otherwise will
block traffic. There can be only one root port on a switch, and there may be multiple
designated ports.
To decide which switch will be the root, an election occurs between all Spanning Tree
aware switches in a given LAN. Every switch has a bridge identifier, which contains
information regarding the switch (generally the MAC address of the first Ethernet
port) and the priority of the bridge. Bridges with lower priority number will have
preference when the election occurs.
Beyond the bridge identifier, each port of the switch has a port identifier, containing
the port number and port priority. If there is a tie in path cost, ports with lower priority
number will have preference in the tree.
The path cost is a number that is used to Spanning Tree aware bridges to decide
which path should be used as a tree branch. If the port has more speed capacity, the
path cost will be lower. The lowest path cost to a bridge will be the path chosen. The
following table exemplifies the recommended cost range to be used:
Recommended cost range of the paths:
Data Rate Recommended cost range
10 Mbps 50 – 600
100 Mbps 10 – 60
1 Gbps 3 - 10