Rapid Spanning Tree
7278_en_04 PHOENIX CONTACT 4-19
4.2.5 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 cable 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.
– 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 2 to 8 s.
– Use priority assignment to specify a central switch as the root.
– It is also recommended to assign a switch as the back-up root.
– For short switch-over times, all switches should support the Rapid Spanning Tree
Protocol.
4.3 Large tree support
4.3.1 Large tree support
If RSTP is operated using the default values, it is suitable for up to seven switches along the
relevant path (see Figure 4-14 on page 4-17 and Figure 4-15 on page 4-17 as an example
for the relevant path). The RSTP protocol would therefore be possible in a ring topology for
up to 15 switches.
The "Large Tree Support" option makes the ring topology suitable for 28 switches along the
relevant path if RSTP is used. The large tree support option could provide an RSTP ring
topology with up to 57 devices. When using large tree support, please note the following:
– In the large tree support RSTP topology, do not use devices that do not support large
tree support.
– Enable the large tree support option on all devices.
– If RSTP is to be activated as the redundancy mechanism in an existing network with
more than seven switches along the relevant path, then the large tree support option
must first be enabled on all devices.
– It is recommended that large tree support is not activated in networks with less than
seven switches along the relevant path.
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. If required, 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, due to the cable interrupt between the media
converters, and if no link down is forced at the switch, timer-based detection is activated,
which may result in longer switch-over times.
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