Stp/Rstp; Spanning Tree Configuration - Siemens SCALANCE X-300 Configuration Manual

Configuration using Web Based Management and Command Line Interface
4.5 The Switch menu
4.5.13

STP/RSTP

4.5.13.1

Spanning Tree Configuration

Avoiding loops on redundant connections
The Spanning Tree Protocol (STP) allows network structures to be created in which there
are several connections between two stations. STP permits exactly one path and disables
the other (redundant) ports for data traffic. This prevents loops forming in the network. If
there is an interruption, an alternative path is found via which the data is sent. The
functionality of the spanning tree algorithm is based on the exchange of configuration and
topology change frames.
Definition of the network topology using the configuration frames
The switches exchange configuration frames with each other known as BPDUs (Bridge
Protocol Data Unit) to calculate the topology. The root bridge is selected and the network
topology created using these frames. The root bridge is the bridge that controls the spanning
tree algorithm for all involved components. BPDUs also bring about the status change of the
bridge ports.
Rapid Spanning Tree
The Rapid Spanning Tree Protocol is based on the Spanning Tree Protocol. With RSTP, the
reconfiguration time was optimized by the devices collecting information about alternative
routes during normal problem-free operation. Typical reconfiguration times with spanning
tree are between 20 and 30 seconds. With rapid spanning tree, the reconfiguration times are
around one second. This was achieved by the following measures:
● Edge Ports
A port defined as an edge port is switched active directly following a link up. If a spanning
tree BPDU is received at an edge port, the port loses its role as edge port and it takes
part in (R)STP again.
● Point-to-point (direct communication between two neighboring switches)
By directly linking the switches, a status change (reconfiguration of the ports) can be
made without any delays.
● Alternate Port (substitute for the root port)
An alternate Port serves as a substitute for the root port. If the connection to the root
bridge is lost, the IE switch can establish a connection via the alternate port without the
delay caused by reconfiguring a connection.
● Filter table
In Rapid Spanning Tree, ports affected by a reconfiguration are immediately deleted from
the filter table. With spanning tree, on the other hand, the point at which a port is deleted
is decided by the time when the port was entered in the filter table.
● Reaction to events
R apid spanning tree reacts to events, for example an aborted connection, without delay.
There is no waiting for timers as in spanning tree.
In principle, therefore with rapid spanning tree, alternatives for many parameters are
preconfigured and certain properties of the network structure taken into account to reduce
the reconfiguration time.
196
Configuration Manual, 10/2014, C79000-G8976-C187-22
SCALANCE X-300 / X-400