Configuration Notes For Rapid Spanning Tree - Phoenix Contact FL SWITCH SMN 6TX/2POF-PN User Manual

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5.2.5

Configuration notes for Rapid Spanning Tree

In contrast to the Spanning Tree method, the Rapid Spanning Tree method supports event-
controlled actions that are no longer triggered based on a timer.
If one line fails (link down), the Rapid Spanning Tree method can respond more quickly to
this failure and thus the switch-over time can be kept low.
A link down or link up must be detected at the switch so that the RSTP switches can detect
a line failure and a restored line more quickly. Please take into consideration, in particular,
paths where media converters are used. It might be possible that media converters offer
setting options to transmit the link status of the fiber optic side to the twisted-pair side.
If a link down is not detected at the switch because the cable interrupt is between the me-
dia converters, and no link down is forced at the switch, timer-based detection is activat-
ed, which may result in longer switch-over times.
– For short switch-over times, structure your network in such a way that a maximum of
seven switches are located in a cascade up to the root switch. The switch-over times
can range from 100 ms to 2 s.
– Use priority assignment to specify a central switch as the root.
– It is also recommended to assign a switch as the backup root.
– For short switch-over times, all switches in the redundant topology should support the
Rapid Spanning Tree Protocol and no hubs should be used.
5.2.5.1 Connecting the switches to form a meshed topology
Having activated (Rapid) Spanning Tree for all switches, you can create a meshed topology
with redundant data paths. Any data links can now be created without taking loops into con-
sideration. Loops can even be added on purpose in order to create redundant links.
In this context, a data path between spanning tree switches can be
– A direct connection.
– A connection via one or more additional switches that do not support Spanning Tree.
If Spanning Tree is not supported by all of the switches used, the reconfiguration time for
Spanning Tree is extended by the aging time of the switches without Spanning Tree sup-
port.
– A connection via one or more hubs that do not support Spanning Tree.
Furthermore, a data path can consist of a connection of a spanning tree switch to
– A termination device.
– A network segment in which no loops may occur, which consists of several infrastruc-
ture components (hubs or switches) without Spanning Tree support.
For the last two data path options, no specific precautionary measures are necessary. If
necessary, you can use the "Fast Forwarding" option for the respective ports (see Section
"Fast forwarding" on page 120).
For the first three cases, the following rules must be observed:
– Rule 1: Spanning Tree transparency for all infrastructure components
All infrastructure components used in your network that do not actively support Span-
ning 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.
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