User manual
Rapid Spanning Tree
7278_en_01 PHOENIX CONTACT 4-9
– Rule 1: Spanning Tree transparency for all infrastructure components
All infrastructure components used in your network that do not actively support
Spanning Tree must be transparent for Spanning Tree messages (BPDUs) and must
forward all BPDUs to all ports without modifying them. When Spanning Tree is
disabled, the switch is transparent for BPDUs.
– Rule 2: At least one active Spanning Tree component per loop
An active Spanning Tree component supports the Spanning Tree Protocol,
sends/receives and evaluates BPDUs, and sets its ports to the relevant STP states.
Each loop in a network must have at least one active Spanning Tree component to
disintegrate the loop.
Example:
Figure 4-8 Example topology
The example topology shown above contains six loops. Each of these loops contains
active STP components, e.g., device 4 and device 2. In this way, all loops are broken by
STP.
– Rule 3: No more than ten active Spanning Tree components in the topology
when using Spanning Tree default settings
The ability to disintegrate any topology to form a tree without loops requires a complex
protocol that works with several variable timers. These variable timers are
dimensioned using IEEE standard default values so that a topology with a maximum
of ten active Spanning Tree components always results in a stable network.
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