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(Rapid) Spanning Tree
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5.1.5 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 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 should not use hubs.
5.1.5.1 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
consideration. Loops can even be added on purpose in order to create redundant links.
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.
– A connection via one or more hubs that do not support Spanning Tree.
Furthermore, a data path can also 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
infrastructure 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 5-13).
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For the first three cases, the following rules must be observed:
– Rule 1: Spanning Tree transparency for all infrastructure components
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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.
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 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 because the cable interrupt is between the
media converters, and no link down is forced at the switch, timer-based detection is
activated, which may result in longer switch-over times.
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
support.