Danfoss MCA 122 Modbus TCP Operating Instructions Manual page 20

How to Configure MCA 122 Modbus TCP
When loops occur, some switches see stations appearing
on both sides of them. This condition confuses the
forwarding algorithm and allows for duplicate frames to be
forwarded.
To provide path redundancy, Spanning-Tree Protocol
defines a tree that spans all switches in an extended
network. Spanning-Tree Protocol forces certain redundant
data paths into a standby (blocked) state. If one network
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segment in the Spanning-Tree Protocol becomes
unreachable, or if Spanning-Tree Protocol costs change,
the spanning-tree algorithm reconfigures the spanning-tree
topology and reestablishes the link by activating the
standby path.
Spanning-Tree Protocol operation is necessary if the
frequency converters are running in a ring/redundant line
topology.
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