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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 Reconfigu-
ration 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-compatible allows 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.