Enhancements in Release F.04.08

Configuring Rapid Reconfiguration Spanning Tree (RSTP)

The IEEE 802.1D version of Spanning Tree (STP) can take a fairly long time to resolve all the possible paths and to select the most efficient path through the network. The IEEE 802.1w Rapid Reconfiguration Spanning Tree (RSTP) significantly reduces the amount of time it takes to establish the network path. The result is reduced network downtime and improved network robustness.

In addition to faster network reconfiguration, RSTP also implements greater ranges for port path costs to accommodate the higher and higher connection speeds that are being implemented.

Transitioning from STP to RSTP

IEEE 802.1w RSTP is designed to be compatible with IEEE 802.1D STP. Even if all the other devices in your network are using STP, you can enable RSTP on your switch, and even using the default configuration values, your switch will interoperate effectively with the STP devices. If any of the switch ports are connected to switches or bridges on your network that do not support RSTP, RSTP can still be used on this switch. RSTP automatically detects when the switch ports are connected to non-RSTP devices in the Spanning Tree and communicates with those devices using 802.1D STP BPDU packets.

Because RSTP is so much more efficient at establishing the network path, though, that it is highly recommended that all your network devices be updated to support RSTP. RSTP offers convergence times of less than one second under optimal circumstances. To make the best use of RSTP and achieve the fastest possible convergence times, though, there are some changes that you should make to the RSTP default configuration. See “Optimizing the RSTP Configuration” below, for more information on these changes.

Note

Under some circumstances, it is possible for the rapid state transitions employed by RSTP to result in an increase in the rates of frame duplication and misordering in the switched LAN. In order to allow RSTP switches to support applications and protocols that may be sensitive to frame duplication and misordering, setting the Force Protocol Version parameter to STP-compatibleallows RSTP to be operated with the rapid transitions disabled. The value of this parameter applies to all ports on the switch. See information on Force Version on page 142.

As indicated above, one of the benefits of RSTP is the implementation of a larger range of port path costs, which accommodates higher network speeds. New default values have also been implemented for the path costs associated with the different network speeds. This can create some incompatibility between devices running the older 802.1D STP and your switch running RSTP. Please see the “Note on Path Cost” on page 145 for more information on adjusting to this incompatibility.

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