Rapid Spanning Tree Protocol

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The Rapid Spanning Tree Protocol (RSTP) is a network protocol for deactivating redundant paths in local networks or reactivating them if necessary (failure of a connection). It is a further development of the Spanning Tree Protocol (STP) and compensates for its main points of criticism. It is defined in the IEEE 802.1w standard.

If, with the STP, if a network component ( switch , bridge, etc.) fails , all connections are interrupted until the new topology has been calculated, then with the RSTP only the paths that ran through the defective component fail. Otherwise the previous paths remain until the calculation of the new topology has been completed. The switchover to the new topology then takes place very quickly. Frequently, the connections that have not failed can continue to exist, since only a few additional ports are activated that were previously deactivated due to redundancy .

With the RSTP, two new port roles were introduced:

  • Alternate ports are blocked ports that connect to a network segment that another bridge can reach more cheaply. If this bridge fails, you can quickly switch to the alternate port .
  • Backup ports are also blocked and connect to a segment that another port on the same bridge can reach more cheaply, for example through a different physical connection (optical port). The software of the bridge determines whether it uses the backup port in parallel or only if the preferred (designated) port fails.

In addition, the sending behavior of the BPDUs (Bridge Protocol Data Unit) has been changed. In contrast to the STP (according to 802.1d), in which a bridge only sends BPDUs to the next bridges in the hierarchy when it receives a BPDU on the root port , BPDUs with the 'R'STP are always at Hello-Time intervals (Standard: 2 s) sent. Expires on a port that is the Hello time three times in a row is assumed that the connection is down to the nearest neighbors of this port. In the case of the STP, the failure could have occurred at any point in the network path behind it.

For fast switching it is necessary that the administrator defines the type of switch for each port when configuring the switches.

  • Edge ports are ports that only connect to end stations (workstations, servers, etc.). No loops can arise there and no BPDUs occur. These ports are quickly switched when reorganizing. If such a port receives a BPDU, it loses its edge status
  • Point-to-point ports work in full duplex mode , so that here, too, switching can be done quickly, as they are only connected to another neighbor (bridge or terminal device)
  • Shared ports connect to a network segment in which further bridges are included. A renegotiation must be carried out in full here.

Fast switching means that the transition of a port from blocking to forwarding takes place in one step, without waiting in between for BPDU packets and values ​​from the neighboring nodes.

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